What size diameter PVC pipe can fit into a reducer labled 2 1/2" to 2"?
If it is what we call a standard reducing coupling then the fitting you are talking will accept a standard 2" PVC on one side and 2.5" on the other. 

There are some other odd ball coupling type fittings out there but for that size of PVC they would be extremely rear. 

When we have a 1" PVC coupling then it is designed to couple together two sections of 1" PVC pipe.  This is basically true for all the individual sizes.  P.