i lost my nut
 

Can anyone tell me the nut size that goes on the alternator?

Which one?

There is a 15mm that holds it on the engine, and I think a 10mm that holds the ground wire on. (could be 13mm)

i thought he meant the one that keeps the pully on.

but i dont know the size

ya ur kinda screwed if its the one that holds the pulley on, your gonna have to take the alty out and go to a local hardware store and get something that fits the threads and is thin enough to fit between the pulley

Its the nut to the electrical connector. the one protected by the red rubber.