Bikes are like clothes. One maker's L may fit nicely, but in another maker's, you need an XL. In clothing, it's the cut that counts, not the letters on the tag. And it's not so different in cycling.

I currently own two 56s, a 57 and a 58. The "contact points"--that is, the bars, saddle and pedals--are in exactly the same position relative to one another on each bike. In other words, despite their being different "sizes", each bike has been fitted to the same measurements, and thus, fits me in exactly the same way.

One of my 56s is a 2006 Trek Portland. When I was buying it, I was riding a 58cm Trek 1000. They each fit me well, despite being two different sizes from the same maker. It's the "cut" that counts.

For the record, "size" refers to only one measurement--the seat tube. Fitting depends more on the head tube (connecting the handlebars and fork) and the top tube. This is why experienced cyclists disregard the size and look more at the specific measurements of these two tubes.

For instance, seven years down the road, I know that a road bike with a top tube around 565mm and a head tube around 170mm will fit me perfectly, assuming 73° head and seat tube angles. That's what I look at, and I can buy a bike sight unseen based on those two measurements and two angles. In fact, I've purchase my last two bikes based on the geometry sheet alone, and have been delighted with them. One is the 57, the other a 58.

Because the top and head tube make such a difference in fit, your position--changing your body angle relative to 90°--alone can swing you between two sizes.

So don't sweat the sizes. Go by your fitter's advice, and how your body feels. Bear in mind too that as you become stronger and more flexible, your position on the bike, and thus your fit, will change.