To convert gallons to liters, multiply by 3.785. Therefore, a 15-gallon tank holds roughly 56.8 liters, and a 21-gallon tank holds about 79.3 liters.
You need to check your owner's manual to determine the exact fuel tank capacity for *your* specific 2003 Chevy S10. The information sticker inside the driver's side doorjamb might also list it.