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eBags provides a reasonably thorough chart of US domestic airlines and their carry-on size limits.
Without knowing which airline you're using, there's no way to provide an absolute answer, but you can get a general feel this way.
Also note that just because you're permitted a bag in your carry-on allowance does not alone guarantee that you'll be permitted physically to carry it onboard. In some cases, usually on smaller, or completely full aircraft, the airline may ask or require certain passengers to check certain bags (usually the larger ones) at the gate.
Generally, they try to work with you in these situations, and will respect your desire to keep certain valuable and/or fragile items on your person. You'll just need to talk to the airline personnel at that time, if the situation arrives.
If it's any assurance, most people prefer to travel with their electronics, and airlines know this, and will generally work with you.
In a worst-case scenario, you might be charged a premium fee for this privilege (I've only ever experienced this on the cheapest of budget airlines).
Upvote:1
It would help to know which airline you will be taking as the allowed bag size is airline dependant. Anyway, I have found a page where you can spot the allowed bag size and weight for a lot of airline companies : http://wikitravel.org/en/Airline_baggage
Upvote:1
One of the other answers noted that bags will sometimes be gate-checked. In my experience, a bag that is obviously small enough to fit under the seat in front of you will never be gate-checked. Airline under-seat measurements vary, as noted, but the folks at DogJaunt.com seem to have done quite a lot of under-seat measurements. (In-cabin pets must be put under the seat, of course, which is why this info is on a pet-travel site.)