The newer tunnel hulls are getting larger, some of the 3.5's are getting up around 34" as the RPM and HP increase. Some of the first ones were only 24" when the first K&B 3.5 came out.
Right now there are only 3 of them to choose from, the K&B, O.S. and the Thunder Tiger is coming back out again. There will be one more this year, hopefully in a few months, the Irwin. There are others like the MAC, Nova Rossi and Rossi engines that fit the K&B lower units.