The British had 4 different lengths of buttstocks for the No. 4 rifle that could theoretically be adapted to each individual soldier. (Lots of luck getting the "right" one though, given the military logistics system.)
Mutilating one's weapon would be a serious disciplinary problem. Maintaining interchangeability would be important, because somebody else might get your gun if you were a casualty. (The British had an official system in place for buttstock replacement, as noted above.)
Generally speaking, soldiers were supposed to adapt to their issued weapon, rather than the other way around. Note that, in the first place, recruitment parameters were supposed to weed out people who were not "normal." For example, if you were a dwarf, you would be rejected. After that, it wouldn't matter. The heavy BAR was often assigned to the smallest guy in the squad.