Tentatively in the market for a new bike. Unfortunately, the brand/model (Kona Smoke) I'm most looking at is only sold in a few less-than-accessible locations in my area, *and* I don't know what size I need. So: I'm wondering how much difference there is in the sizing of bike brands, and if it's fine to get sized on some random brand (say, a cannondale or specialized) of comprable model so I have a general idea of what size I need on the bike I actually want. (my current bike is a 15" framed specialized hardrock, and is too small for me, but I suspect I only need the next size up --on specialized that's 17", but on Kona that's 18").