M.,
I believe for checked baggage the major concern is weight. if you are an inch or two over I think it's really a non-issue for them.
carry-on bags are a bit different though. My mother in law is a flight attendant for United and she says you would be amazed at what people try to bring on as 'carry on' - like those big hockey gear bags.
Also,... your purse counts as a carry on. This really irritated me because my purse was so small it would not even fit a pop can inside... I ended up having to shove it inside my carry on because I also had a shopping bag and my carry on bag (i always pack my carry on with my essentials like toothpaste, toothbrush, change of clothes, hair brush, make up...in case my luggage gets lost, at least i can change my underwear and brush my teeth!)
anyway... I don't know if i helped at all - i'm still working on my first cup of coffee and so i'm a little scatter brained jsut yet ;-)
Enjoy your trip!