Question about carry-on size?

Alright, I'm flying for the first time soon, and I don't want to have to check any baggage. I was thinking of buying a quilted duffel bag as a carry on (seems less chunky than a hard sided suitcase). This is what I was thinking about: a href="http://www.whitneycarolinedesigns.com/monogrammed-duffle-bag--red-polka-dot.html" rel="nofollow"http://www.whitneycarolinedesigns.com/mo…/a But it is slightly over the size they say you can bring on (but only by 3 inches or so). I understand the size issue is because you'll need to store it in the overhead compartment, but if it can be squished in there, will they really make you check it if it is slightly too large? I'm traveling on American Airlines, if that helps any...