I heard about sizer enforcement earlier in the year but did not see it happening in the airports I use (primarily SFO and EWR.)
However, I have seen enforcement this summer as more families are traveling. One one flight two weeks ago the gate agent stood at the door of the jetway solely to look at people's bags as they were boarding. He didn't size check mine (Tumi Tegra Lite Continental) as it is clearly one of the smaller rollers. But later in the flight, I didn't see anyone trying to fit huge rollers in the overhead and then having to gate check, as I would usually see.
They size checked at boarding both ways on my round trip this week, again giving a pass to anyone with one of the smaller rollers.
Frankly, I'm happy to see it. I disklike having the boarding process delayed because people roll on items that won't or barely fit, or seeing people get shut out of overhead space toward the end of boarding.
I'm traveling with my family in August, and I bought them new 20" rollers to be sure we're compliant.