Our general rule of thumb for garage lighting is: Choose fixtures that are a quarter to one-third the size of the garage door's height. (Source) Erring on the side of a larger fixture size is a good idea too, since you don't want the lights to be so small that they're out of proportion.

At the source I linked to, there's more info about different garage lighting configurations depending on how many doors you have and so on. Check it out and let me know if you have further questions.