Touring - That's really huge man. . . Trek 520 - 25 inch

Spent a few minutes at the local bike shop looking at their Trek 520's. They had one with in a 25 inch size. It was right next to a 17 incher. The thing that surprised me the most was that the larger one had less than 1 inch longer wheel-base. I've seen some cycles vary by as much as 2 or more inches in length.

When I was shopping around what really surprised me about the 520 was that the wheelbase and chainstays seemed pretty short for a bike touted as a loaded tourer.

I wasn't specifically looking for a tourer, but did want something that would handle large panniers without catching my heels.

I finally settled on a 7300 hybrid, which has a longer wheelbase than the 520 and chainstays that are only half a centimeter shorter than those on the 520.

John

Bike sizing always seems a bit strange at the extremes of the range.
Big bikes are always short and built low off the ground. Small bikes are always long and built high up. Really they are just taking a medium sized bike and tweaking some of the measurements, but not in any proportional way. Just compare the ratio of TT length to standover height, look how much the bottom bracket height varies, and consider how proportionally sized cranks would fit into the frame.
Big and small riders are really let down by mass market builders, and have to go to custom shops for a sensible solution, but even here, not all custom builders are experienced at extreme sizing.

the trek 520 has the same bottom bracket height, 10.4 cm, on each of its bikes, a height that isn't particularly high compared to other road bikes.

If you are willing to spend a little (or a lot) extra, there are a few frame makers who make their frames proportional. That is, the geometry stays the same as the bikes get bigger or smaller.

Co-Motion makes a couple of touring bikes, the Nor'Wester is more of a sport/touring/commuter bike, while they have another that is a dedicated touring bike.

They run around 3 grand for a complete bike, but they are nice!

Panasonic used to make several really large frames. 27" frames were available straight off-the-shelf, and even 29" frames could be ordered.
As far as the wheelbase is concerned, well, yeah, they should be similar, since the chainstays are the same length. The only thing that will make any difference is the top-tube length, plus a tiny bit from the lateral component of the longer headtube. Larger framed bikes tend to have longer reach stems, so less of the difference is in the top tube than you might think.

If you put longer than normal cranks on, and the rider has big feet, the rear extension may be a few inches more than a medium sized rider. The heel will probably strike the panniers. You also get the potential for toe-clip overlap.

The bottom bracket height should be chosen with respect to the crank size. Cranks range from 140mm to 190mm (a 5cm range). The bottom bracket should be high enough for cornering clearance, but no higher. Using the same height for every size is a design flaw.
Big riders need big cranks, so the frame should be sized accordingly.