Just starting to work on my own bike, so I have a basic question.

I just bought a Marin Belvedere. The left crank arm was off when I bought it. It has Truvativ IsoFlow cranks (square tapered spindle). From the reading I have done, I understand that these are bad cranks, and should just be replaced (probably replacing the bottom bracket with something besides square tapered).

But for now I just want to get the left crank back on. The bolt for that crank is missing. How do I determine what size/type crank bolt I need?

Thanks for any help.