Prope,
Frames are simply the interior framework for your hull. As to size of your airboat, if you are building the "swamp" style, a ratio of 1.5:1 is about right. some use 1.6:1. 5" is kinda tall for your hull. 1.5" to 2" is more common. Tapered sides will help your boat slide when cornering and help prevent rollovers. Hope you find this helpful. You can find lots of help over at R/C Airboats as well as here in this forum. Best to ya!!
Gordy