Bag sizer
My carryon is wheeless and soft sided and, in a test pack, can be squished to UA allowable size, 22 x 14 x 9. My personal item bag is also soft sided and, although it looks big, can be squished to 17 x 10 x 9. I'm not trying to game the system; I just don't want any surprises at the gate.
The only time I have ever witnessed a problem was at the gate of another major US airline. The flight was full and the gate attendants were looking for any reason at all to make passengers gate check their bags. They were borderline rude about it. I understand in a way after seeing people carry on huge suitcases clearly not meant to be overhead. I've seen people nearly break the overhead bins trying to force an oversized suitcase up there. I've also seen people put both their carryon and personal item overhead, not polite at all.
I wish I could pare down to what fits in an allowable personal underseat item.