What size is drain plug on 1996 Chevy s10?

The drain plug size for a 1996 Chevy S10's differential will vary depending on the axle type. There's no single definitive answer. It's commonly either 10mm or ¾ inch. You'll need to check the drain plug itself to be sure, or consult a repair manual specific to your S10's axle (e.g., 8.5-inch 10-bolt, 7.5-inch 10-bolt, etc.).