You can easily tell what size it is just by measuring it. Take a ruler, and hold it up to your bottom bracket shell. Measure the width of the BB. shell, but only the shell, not the bottom bracket that is in it as well. It should be either 68 or 73 mm wide. Next. measure the spindle length, you will need to take your crank arms off to do this. It will probably be either a 110, 113, or 118 mm long spindle. Last thing you need to do is determine the type of spline. If you have shimano cranks, go to www.shimano.com If you have truvativ cranks, it is probably an ISIS spline, and has 10 splines on it. Also, taking the bottom bracket out of the shell on the bike will usually tell you what size it is, because they typically print it on the bottom bracket.