All of the DYKWIA's attitude aside, I recently had a GA-in-training tell me I needed to pink-tag my computer backpack on an RJ. I told him no, it will fit under my seat (which it will, with room to spare). When he tried to insist, I
may have muttered something under my breath about having been on more RJ's in the past two months than he had been on in his life, but I don't exactly remember.
Of course, I was actually polite. I told him that if it didn't fit, I would pink-tag it.