I Bought the medium 2.5" set of two as a test for my iFetch LARGE. They worked! But they get very slobbery and wet like regular dog 2.5 tennis balls (and the crappy blue ones iFetch sent in the box and were destroyed immediately).  I have also tried the rubber version of these which say they are 2.5" but the iFetch says otherwise! They are a tee-tiny bit too small. SO.... if you want better balls for the iFetch LARGE, the medium 2.5 do work and once they get slobbery the iFetch won't throw it as far. I trade out the wet one fore a dry one. After that... we're done until the balls dry.