I am thinking about scratch building a nitro boat. I want to make it as small as possible but still big enough so that it doesn't have to be completely flat calm to use it. I was thinking along the lines of a .16 to .21 ish. Does anyone have any advice on the best type of hull to make and what size would be the minimum. Also what materials would be best, I was thinking of fibreglass.

Take a look at the Wild Thing designed by John Finch in R/C Boat Modeler. It's a .21 powered boat that seems to handle Non Perfect water conditions well. If you have any ability at all, you may be able to scale the boat up to .45 size engine if a ,21 engine is not suitable. You can find pictures of mine, elsewhere in this forum (wild thing construction).