It varies from not strict at all, to extremely strict, depending on whether it's a legacy carrier like AA or UA, or a low cost carrier like Spirit or Easyjet. Since charging for carryon is part of their model, they're extremely strict. The personal item has to be able to fit under the seat in front of you.
Does American Airlines actually check carry-on size?
American Airlines carry-on dimensions should be no more than 22 inches X 14 inches X 9 inches (56 cm X 36 cm X 23 cm), including handles and wheels. All carry-on must fit the sizer at the departure gate. If the bag does not fit in the sizer, it will need to be checked.
How strict are carry-on size limits?
How do they check carry-on bags size?
Carry-on bags are limited to 22 inches long, 14 inches wide and 9 inches high including wheels and handles. All three measurements shouldn't add up to more than 45 inches.
What if my carry-on is one inch too big southwest?
Your carryon can be no bigger than 24” (L) + 16” (W) + 10” (H). If it's bigger than those dimensions, it will need to be checked. Where can I store my carryon? You must store your carryon in the overhead bin.