Different sized pupils is called 'anisocoria' and the causes are multiple, from injury to the eye to more serious diseases of the brain.
If your cat isn't in pain or distressed, then it doesn't sound very urgent, but best take him to the vet for a checkup just to be sure. If he isn' opening his eye fully, it could be because it is sore.
Found this website explaining it a bit more:
http://www.petplace.com/cats/anisocoria-in-cat s/page1.aspx
Let us know how you get on.