the stockers were 25, i gained that much over stock size, there close to my 26 itp xtr, about half inch in differance, and i only run mine at 6 psi, My atvin buddie has some and they run 15psi in them and there like 2 inches taller then mine,

 

i would not upsize , ecspeicly on a 500 popo, my atv buddie same as the one befor has a 05 500 popo, she put 25's (i know what you think lol) her rpms are faster so take off and acceleration are way better, if ur bike uses 26 , get a 26(since GC is needed)