FIRST, you need to find a crankset that matches your bottom bracket...

Then, you'd have to check the specs for the crankset you're looking at to find the BCD, then you can start hunting for chainrings.

If you're shopping for cranks from parts bins, bring a metric rule with you and a cheat sheet for the adjacent bolt hole center multipliers found here..

Ahh, I might as well paste it too, why not:

Take the adjacent bolthole (center to center) measurement and:

Multiplying this dimension by 1.701 will give you the actual BCD for a 5-bolt chainring.

Use 1.155 for 3-bolt chainrings, 1.414 for 4-bolt chainrings and 2.000 for 6-bolt chainrings.

Crankarm length is pretty easy to figure out, center-to-center pedal hole to spindle hole.