As per this post
they have the same diameters, which should be 17mm.
If Wiseco are making pistons for these bikes, I want to see their skirt length, ring thicknesses and valve clearances before I'd put my money down for one. The CBR100RR might share a few things with the 250R, but with that tight of a clearance, I don't want to take chances with the valve relief angle or the weight.
From my experience, the stock 1000RR piston should be good to go. We've run it on a car engine which has perhaps a little better harmonics than a single cylinder bike engine (still an inline 4 though). As long as you have a good tune, the manufacturing quality of a OE Honda piston should be able to take the power. It is usually the connecting rod which either bends or breaks due to detonation. If someone can share a pic or two, along with various diameters and widths, I can build it in my simulator and do a FEA analysis on it to determine how strong it is. I have specifications of various forged and cast connecting rods in my database, so a comparison should be possible.