Comentário oficial
Thanks for the info, Nathan.
Unfortunately, there is no dedicated loop shortening/extending hardware button(s) on your DDJ-RB. You can, however, assign keyboard shortcuts to "Halve the loop" and "Double the loop" on each deck.
Preferences > Keyboard > Select Deck 1 or 2 > Scroll down to the "Halve the loop" and "Double the loop" options and assign any keyboard shortcut you like.

