for example, http://www.bikesdirect.com/products/windsor/windsor_ghost6900.htm, this bike has small, medium, large, etc. options. what would you recommend for a smaller guy like myself-- ~64", 140lbs? thanks!
You might be on the cusp of s small and medium. When I was faced with this with my Intense Tracer VP I went "up" to the medium and then used an 80mm stem to shorten the bar/seat relationship. I like a bike with a steep seat tube angle that puts me "on top" of the crank/BB and not behind it. I am 66" tall...32 inch arms and a 29 inch inseam. I ride a Medium Intense Tracser VP and a 16" Kona Hei-Hei 29er. Look up the Tracer VP and compare its specs to the Motobecane... should help you.
kinda late on this but its harder than picking a size based on height. many frames have different geom and the top tubes differ in angle. for mtb you want a couple inches of clearence on stand over height because your on and off the bike allot. so its hard to say without actually standing over it
size matters As the previous person mentioned, different manufacturers have different geometry that can make "fitting" a challenge. Depending on manufacturer, your probably a medium or maybe a small. Most quality bike shops can help with fitting. This is well worth it to spend the money to make sure you have the right stem, seat and pedal position. Even with mtn biking where you will spend a fair bit of time off the saddle, there are still plenty of in-saddle spinning times and having the bike adjusted to your personal dimensions helps.