a_a:

I used a number of sizing charts (btw, I bought a Gravity Zilla) that can be found on the internet (using google). I then compared those to the detailed dimensions that are shown for the Gravity Zilla. I did not just use the simple sizing chart showing only height.

I ordered the 53.5 because most sizing charts indicated 52 cm to 54 cm - I am 5 ft 11 in, have a 31 in inseam, but a long torso. Bikes Direct warns strongly about the standover height being at least 1 to 1.5 inches lower than your inseam. Although I did not ignore this, standover is not as important to me as effective top tube.

After all this (where the 53.5 I ordered was technically too high for my inseam, but the effective top tube was in the ball park) the Gravity Zilla I ordered is right at my crotch (no gap in standover height), but the stretch, from seat to bars is just right. The fit, in total, works well and the bike is responsive and comfortable (I did have to hack saw the seat post - the adjustible part that goes into the seat tube - for length - because it would not drop down internally past the water bottle bosses).

All this to state, don't just go with the "simple" height to size stated on the website, do your research on fit, compare with the detailed dimensions of effective top tube, and compare this with seat tube lenghs. If all this is confusing, and you don't want to do the research, consider buying a bike at your local bike shop. This will help you avoid disappointment and repacking the bike exactly as you received it from Bikes Direct, along with the cost of reshipping.