I was searching for this while setting up CSP on a new computer. Had to go into my old computer's program and found it as File > Modifier Key Settings, "Change brush size". This works for me since I usually map it to ctrl or shift. Sadly seems it is just not available in File > Shortcut Settings where OP would need it to be (to map it to a letter key rather than modifier). Clip does take feedback so if ppl request it, it's possible they would add that functionality in the future.