yep, what those guys said. What kind of bike, there are generally some certain stds for example a road triple is 118mm A MTB triple I believe is usually 113. A compact road is 110 and I think a regular road is whatever the other std size usually is. Generally, your crank will tell you in the instructions. try googling it, most brands have their specs available via pdf on their websites.
Of course, we could always ask you what the hell yo udid with the old one? if it's a modern bike with a cartridge type bottom bracket it should be printed right on it.