Tom
I think I used 1 1/4" to 1 1/2" and they screwed down tight.... figure .125 for the piece and .375 for the wood wall and .125 for the stud.......
We drill a 1/8th inch hole then use another drill with a magnetic hex driver and screw it in to the stud.... .....
I would plan out what you want in the trailer before you start drilling and lay every thing out based on the stud locations....
You might want to check out
www.pitpal.com
got all kind of trailer stuff...
Morris