If you want an exact size measure the space between the wheel wall and the fender flares in MM and get the spacer closest to this number that doesn't go over it, because someone with that same tire size on a different wheel might get a different size depending on the offset of the wheel they have and viceversa with same wheel different tire setup. So measure your gap to get your ideal size, also I don't think spacers will cause understeer since the front ones are usually thin ones.