I'm buying a BB and I'm wondering how they are measured so that I can be sure it fits my frame. I don't have any documentation for my frame so I can't know and I am having trouble finding anything online.

Thanks,
Patm1313

measured by the BB shell, that is how wide the frame is where it holds the bb, usually 68 or 73mm, some frames are less and use spacers, (like my cannondale) some 73mm bb come with a spacer to use with a 68mm shell, there also exists a 71mm bb shell, but that was on old french road bikes, so shouldn't be a problem if you posting in the mtn bike forum, those where also threaded differently

what frame is it, maybe someone here knows the size

It's 68mm across, but on JensonUSA I'm seeing another measurement, somewhere around the 115mm range (ie. 68x126mm) what's the other measurement?

oh yeah thats the spindle length, sry my mtn bike has external bb so no spindle so i didn't think of it, basically if you measure the longest piece of the bb, the part the cranks go on (the spindle or axle) measure from end to end

To figure out what spindle length you need, you'll have to check the documentation/specs for the crankset you'll be using. The crankset specs will also tell you what type of BB you need: square taper, Shimano Octalink version 1, Shimano Octalink version 2, ISIS splined, external bearing cups, or perhaps some other (new) type.