All 82 to 04 S-10s and Blazers take 4x6s in the dash
All 82 to 04 S-10s take either 4x6s or 4x10s in the rear. Just look at it, if its a huge oval, its a 4x10, if its oval but closer to a circle , its a 4x6.
All 83-94 2 door Blazers use 6x9s in the rear
All 83-94 4 door blazers use 6 3/4s in the rear
All 94-04 S-10s and 95-04 Blazers use 6 3/4s in the front doors
All 95-04 Blazer 2 doors use 6x9s in the rear
All 95-04 4 door Blazers use 5 1/4s in the rear
there are a few variances of this. some blazers come with tweeters in the dash. the info above will hold true 90% of the time, but it's always safest to check for yourself.