If a S10 came from the factory with vacuum assisted brakes, regardless of year, the rear wheel cylinders where 3/4" bore diameter. This is the same size as all g-bodies.
If you order wheel cylinders for a manual brake equipped S10, they will be the larger 7/8" bore diameter wheel cylinders. If you buy them at the local auto parts store, make sure you measure the bore for each wheel cylinder before you buy. They may box up a set of 3/4" bore diameter wheel cylinders but advertise them as 7/8" bore diameter wheel cylinders.
As far as I know, these wheel cylinders can only be used with the 3 bolt axle flange that came on all g-bodies, most 3rd gen f-bodies, most S10 trucks and SUVs, and maybe the Astro Van.