T-top for 18' SC

[NoBones]

You have to go with what works for you. There is no rule of thumb on size. Yes I understand you do not want to look goofy for the size of the boat. What type of fishing
(if you fish) do you do? do you dive? or what ever your general purpose of the boat is. Do you want your electronics over head? Take all those things into consideration and discuss it with a good fabricator. Look at other boats, take measurements and pictures. It will all fall into place.
Good Luck, See ya, Ken