Long pipe about 5-6 inches diameter and make it too long to start with and then cut small pieces untill it it sounds good:).
I tried on my own old boat with a "normal" length but to loud,made an extension that i fiddled with in length untill i was satisfied.
Bigger diameter pipe will make the Hz lower,longer the same.
Try to have as low restrictions as possible on a two stroke.
also remember that all the oil that follow the exthaustgases will finaly burn so better to have oil on deck or clothes than in a silencer.
When you have your first exthaust fire its an experience...........
Edit:on my little 15hp Seffle i had a 4"pipe 5 feet long and the Skandia 120 is a "bit" bigger in deplacement.