saturn v rocket 1/96 scale

I last did this kit as a young teenager who wanted to get it finished ASAP, so can't give any 1st hand advice, however you should have a look at the Apollo Maniac website as it is just bursting with Saturn & Apollo information. Have a look at these links;

For general Saturn model info;

http://apollomaniacs.web.infoseek.co.jp/apollo/revelle.htm

For specific 1/96 Saturnbuild info, including what you mention:

http://ricksternbach.com/SatV/Saturn_V_Clinic.html

I generally buy the kit if available, and use detail from the kit.  Also, since I have the kit, I tack it together and measure to see what size I need.

Otherwise, scale drawings and a desired scale allow you to figure out what diameter you need (I assume you want the OD).

Unfortunately, tubing does not always come in the size necessary to build a given rocket in a given scale. In that case you must either use another scale if you want to use off-the-shelf tubing, or else use formers and skin instead of tubing.