I took the grips off and moved the brake levers inboard, and road around my fairly large driveway with my hands at different positions trying to decide... then I wrapped some masking tape around the bar ends, and used that as a guide for my hacksaw.... hey, the tape was 1" wide, snicker. filed the cut ends smooth and rounded off the edges, installed the grips, and went, 'perfect!'.

actually, I sometimes think I could take another 15mm off each end, but I dunno, its OK where it is, and i couldn't put it back if I overcut it... FWIW, I wear a size 46 jacket (eg, 46" chest). 21" (550mm) is probably from about the middle of one shoulder to the next, a bit more than armpit to armpit...