how much baby aspirin should I give my dog?

[ஐ♥Daniela♥ஐ]

My dog is about 2 1/2 years old, and wieghs about 45 pounds. A vet once told us to give him baby aspirin when he's sick (and yes, he is sick), and it worked. But the problem is, I forgot how much to give him. How much do you think? PS yes, this is safe, we've done it before!