Three same size jeans, but only one fits

Buying clothes according to a seemingly arbitrary set of numbered sizes can be baffling and frustrating. So, armed with a tape measure, we find out what fits where and why.

I am standing in the changing rooms of New Look in Dubai Mall. Around my ankles is an indigo-coloured pair of skinny jeans, size UK 12. Trouble is, they won't go up much beyond my thighs, which is irritating because I generally consider myself an average size 12. So off they come, and on with another pair, also size 12. These go on and, mercifully, I can get the zip and top button done up. They're almost roomy. But a third pair, the same size again, refuses to be heaved any higher than mid-thigh. So panting with effort, I peel them off.

And yet, in a nearby Marks & Spencer changing room, I can pull on a pair of size 10 Per Una jeans without much huffing or puffing. And a size 10 T-shirt, along with a size 10 cashmere jersey. Odd, one might think, that you could be one size in a certain store and a very different one elsewhere. But actually, it's not odd at all. It may be down to "vanity sizing". Also referred to as size inflation, this is where stores flatter us, the vain customers, by ensuring that size labels remain the same even though clothes measurements are actually on the increase. Bodies are generally getting bigger, but no one wants to have to take a bigger jeans size than they used to.

Earlier this year, Marks & Spencer insisted that it hadn't changed its block sizes (the ones given to suppliers), but it had "tweaked" sizes on its website so that they were based on the "average body". Thus my changing room experiment makes more sense. In Marks & Spencer, the average body is your more mature shopper; in New Look, it is a skinny teenager. Naturally, I will fit into smaller sizes in the former, and have trouble with the same in the latter. But does this mean I'm sized 10, 12 or 14? Where does this leave me? Where does this leave hordes of confused shoppers the world over?

According to Nicole Robertson, the fashion and buying director for Boutique 1, sizing is always a nightmare. The "bane" of a buyer's life, she says. But this isn't necessarily all the fault of vanity sizing, rather that different labels all tend to vary slightly. So while you might think you're a 12 in one label, you may have to take a 14 in another. "Usually Italian sizes come up quite small, and French ones. But honestly, it depends on the designer. We know, for example, that Giambatista Valli comes up teeny tiny. US and UK sizes always tend to be more generous, and it's the same with shoe sizes."

Another that Robertson says tends to come up quite small? Victoria Beckham. "And customers do buy small," she says of Beckham's fashion label. "They want that really tight, sexy fit. It's not very forgiving if you're a 14, although her collection is building and she's adding more depth in terms of style." She's not wrong on the sizing. I take out my trusty tape measure and run it around the waist of a soft, green felt Victoria Beckham dress, newly arrived in store as part of her autumn/winter 2011 collection. It is UK size 12 and the waist is 28 inches, a smallish 12 by today's standard. A case, perhaps, of vanity sizing on the designer's part by catering for the skinnier end of the market?

"If there's a designer that we know comes up small, we won't do the first size," she explains. "But it's a tough one in Dubai because it's such a diverse market. You have to buy across the board for everything from size zero to US 14 or 16." Robertson's job is further complicated by the fact that she has to look carefully at each Boutique 1 branch when it comes to which sizes go where. "With something like Missoni, the shop in Dubai Mall is screaming out for size 38, and a Missoni 38 is quite small. But they're screaming out for them because they get more Russians coming in."

It's a similar story with menswear. "When you get the Russians coming in, this is when you buy the XXL, because they're quite big guys. Then you get the other side of the spectrum, when you're buying for the smaller guys, the Egyptians maybe. The buyer's job isn't easy," she says, laughing. "I get equally frustrated when I'm looking; I always just take what fits. I don't really look at size." The challenge at Boutique 1 is particularly great because, unlike at New Look and Marks & Spencer, it stocks 150 brands, which makes dealing with differing sizes all the more difficult. The same is true for Bloomingdale's, Dubai Mall's branch housing multiple brands from a number of different countries. In another size experiment there, I try on a Matthew Williamson dress, a Moschino dress, a Diane Von Furstenberg wrap dress and a pair of Ralph Lauren linen trousers.

It is an exercise that proves how difficult it can be to shop when you are confronted with a bewildering array of different sizes. French? Italian? American? British? One of the Bloomingdale's staff herself had to pull a small, laminated conversion chart from her pocket to assist me. The linen trousers were US size 8, or a UK 12. I could pull them up, but buttoning them would have meant cutting off the blood supply to my stomach. The Diane Von Furstenberg dress, on the other hand, is also a US size 8 and perfect. Hello curves. The Moschino bubble dress is an Italian size 44, or a UK 12, and won't do up. But I have more luck with the Matthew Williams number, in UK size 12. Across four labels, therefore, there was a pretty wild size range. A Bloomingdale's spokesman told me there was no great discrepancy in sizes from country to country, but this seems inaccurate.

I quiz a wise, older colleague on where she finds the sizes are about right and she replies instantly "Marc Jacobs". This makes sense, given that Jacobs himself is something of a champion for realistic sizes with rumours of a plus-size range in the works. In his diffusion store, Marc by Marc Jacobs, I try on a medium, floral dress from his spring/summer 2010 collection. It fits in the right places. It looks good. Hurrah. Perhaps not quite Dh1,400 good, but it is cheering to leave a store without feeling obese and depressed.

Ironically, the shop in which I do most of my shopping is also where I find sizing tricky. It's Zara. I work in Zara skirts, wear their dresses on the weekends and often flop down on my sofa in Zara tracksuit bottoms at the end of the day. But my wardrobe is littered with items from the store in sizes from small to large. One pair of trousers in medium, two in large, one cardigan in medium, one large waistcoat, a small T-shirt and so on. It's complicated, too, by the fact that in Zara certain things are labelled not in small, medium or large but in European sizes. According to them, a EU size 36 is small, size EU 40 equates to a medium and EU 42 to a large. I have developed the trick of squinting at things on the rails in an effort to size them up before trying them on.

Generally, says Anne Villabra, a UAE-based Zara product manager, sizes come up small because they're based on petite Spanish women. So Villabra says they often request greater stock of bigger sizes from Zara's Spanish warehouses for their stores out here. "We need more extra-large and for the trousers we get 42, 44 and even 46," she explains. Zara shoes are a constant source of bitterness to larger-footed women, because they come up in tiny sizes. I am a UK size 8, but can barely wriggle my toes into a pair of green satin heels in a 41. Do all Spanish women have chic, tiny feet too?

"Yes," says Villabra, laughing, but she adds that people are always asking for bigger shoe sizes in particular so they might be revised soon. Here's hoping. That, at least, is one size increase I'd welcome.