Come on, 99% of the carry ons that people take on board probably wouldn't fit in the sizer, but they fit in the overhead. It's not like he was trying to take a clearly oversized checked bag through. It is still a carry on. GA was on a power trip.
I'm assuming that the bag didn't actually fit in the sizer? Enforcing the rules is not a "power trip". The GA was doing their job. It's not like they were making up their own version of the rules.