Depends on the version you are using, they mostly use 8mm id shimms all around.
I bought a set from Kyosho, with 10 of each, .1, .2 and .3 mm. some .05shimms would be nice too, i don't think they need the .3mm, since they are produced with pretty tight tollerances. (at least the ones I had, which where the 1st gen)
Key to perfect shimming is to make them as tight as possible, without any noticable gear-rubbing..