I just picked up a replacement tensioner pulley for my 94 SE. It is noticeably smaller than the original. The belt will still fit but should I use it or not?

You will want to replace the pulley'* with the same size pulleys. If you replace with smaller pulleys the tensioner will try to take up the slack and may not provide enough tension. This would cause belt slippage and premature wear of the belts. Could lead to a belt falling off and other problems.

^^^^What he said.

Take the pulley back where you got it, but before you do, measure the diameter of the one on your car so you can tell them and get the correct one this time. Your local parts counter guy may have misunderstoofd which pulley/year you asked for.