Boom length is more about sail size, rather than a paricular boom for a particular discipline. e.g wave booms usually have a max length around 170-200cm, slalom booms a max length around 220-250cm and race booms, well, BIG. Back ends can vary a bit if you want to use adjustable outhauls (usually used for racing). With alloy booms, you never really want to extend to the "max" measurement, they get a bit flexy - you always want a little bit of spare extension.

Oh, one more thing. The grip on booms is different diameters - eg 28, 29, 31, 32mm. I prefer to use the narrow grips 28 or 29mm myself, find them a little easier on the hands and wrists.

Hope this helps...

Pugs