The diameter of the roller is set up such that the tapes come out of the guide chute and meet the compaction roller at a tangent (or nearly so, the conformability of the elastomer can change that somewhat).  The outer diameter of the roller is and the bore (for the shaft) is 8 mm.  That said, you should be able to vary the outer diameter by a bit, but there are a couple of things to keep in mind: 1) you do want the tape to meet the roller pretty close to tangent (not just hang in space in front of the roller, possibly having to bend over a relatively sharp radius as it exits the guides), and 2) when compaction loading is applied to the roller, you need to make sure that the tape guides to not drag on the part (if the roller is too small in diameter, the final tape guide will not be above the level of the roller, so the guide will contact the part surface before the roller).