It's so hit or miss, though. There are all these horror stories about being 0.00001" over the limit and having to pay, but I've been on plenty of flights where people walk on with full-sized suitcases clearly not designed as carry-ons and don't have a bit of trouble. I don't think there's any standard consistently enforced across F9, so a lot of it depends on what kind of day the GA is having.

Personally, I use a small travel backpack with fully loaded dimensions that comply with "personal item" standards and a properly-sized carryon because I want to comply, but there seem to be a LOT of people who push their luck.