If it's pulley size that the OP is talking about... their size won't affect speed by much. But the size will affect the shifting and cassette cog max size capacity. A larger guide (upper) pulley will reduce the max cog size the der can handle. Either or both pulleys swapped for larger ones will have the same effect as to have shortened the chain. So the big/big cog combo might need another chain link or two to still work. Also as the chain's path through the der's cage changes it could contact the cage or the tabs. Andy.