Varies by employee. In GENERAL, I haven't encountered rigorous enforcement of carryon size, especially now that most mainline has bins that can fit bags larger than the rules allow.
Odds of an objection go up if the flight is full, the equipment has smaller bins, you're lugging a bunch of stuff, you're egregiously oversized, or you've managed to irk the staff in some way.
Also be aware that some airports limit baggage going through security to carry-on size, or at least have signs claiming they do (never seen it happen personally.)