Why not contact the manufacturers? Bilstein, FOX and King all offer 2.5" and 3.0" shocks. I would also consider cost of each and of course ease of maintinence,parts availability.
i guess that's really how i should have phrased the question - does a 3" fox have
a 2.75 bore etc. just thought maybe someone who rebuilt a lot of shock might
have info on the amount of shock oil they use on one verses the other that's all...
...now let me see Pi multiplied by the radius squared.