Well Ashiko, one thing to make sure is that the tiller fits snuggly (no gaps) on the mount. Very important. Don''t know if your buying a wooden or metal tiller. But if your getting a wooden one then don''t forget to buy the metal casing for the mounting end of the tiller. It looks like a U that is bolted on the end of the tiller. A friend of mine baught a boat that had a tiller minus the metal casing. Well, when we were on a 25 degree heel, the tiller snapped! None of us new any better then but sometimes experience is the best teacher. As for length and mass. Length is a comfort issue, get a length that is comfortable for you. The mass of the tiller depends on the size of the gap you have on the mount. If for example you have a 2 1/2" gap on the mount then the thickness of the wooden tiller on the mounting end should be 2" PLUS 1/2" for the metal casing.
Take measurements of your mounting gap and this will tell you which design to use from the company.
Hope this helps out.