Once you have worked out how much of a gap you have between your axle and your dropout you just get that size and plug it in there. The slammers come with two sizes in one to leave a little option with play to you, they start at the 0.5*1.0mm and go all the way up to 7.5*8.0mm. They are done in Aluminum and the weight is next to nothing. For 14mm axle.
2 wheel slammers for this price!