Two things to consider:
Butt size (we're talking leader here!) should match the line tip, i.e. when leader & line are connected, put a curve in the junction & it should be a smooth curve rather than a sharp hinge. This aids smooth turnover of the leader.
Tippet size should match the fly size but also the weight of the rod, i.e. using a very heavy tippet with a light rod such as a 5wt puts the rod at risk of breakage unless you're very careful.