Do not save too much on this component. Should be adjustable in length, at least for beginners, embrace all the sails, for example, held a 165-210 cm extension is usually enough for sails from 5.0 to 7.0. When you have this, your boom will be until breaks.
Small diameter (for girls)

From a few years more companies specializing in this component, create boom tubes with small diameters. It 'something that must not be neglected just because the diameter allows for less fatigue.