To harass, bully, or fight someone who is big, strong, or skilled enough to put up a fair fight. Often issued as a direct challenge or invitation for that person to fight oneself instead. Hey, that kid's, like, five years younger than you—why don't you pick on somebody your own size! Our law firm is going to represent this group of customers pro bono. If these giant corporations think they can walk all over common folk, it's about time they picked on somebody their own size.