yeesh.....now that's a question GUARANTEED to get you every possible answer....

what you want to do is figure out what magnification you're working with and what distance you want to be when working at that magnification...that will determine the focal length.

for example--if you need like 5' to get enought distance to put in lights...and you need say 1/4 lifesized, then you want (1+1/(1/4))f = 5' or f=1' = 12" = 305mm. but this will require a bellows draw of (1+1/4)*1' = 1.25' = 15" = 381mm. If you have the bellows draw then go for the 12" lens, if you don't, then you have to do some compromising such as have to work at 4' distance or something like that.

just use the formulas (1 + 1/M)*f = working distance and (1 + M)*f = bellows draw.

note--you MAY want to us a shorter lens for more depth of field or a longer lens for less depth of field also--there are a million ways to pick'em.

just figure out what's important to you looks wise or setupwise, etc....and see what that gets you.