Many airlines now have a contraption at their check in area that you try & stick your carry on bag into to see if it fits the requirements.
Personally, I wouldn't mess around with trying to "get by" the size requirements... Cause I've often seen folks get all the way thru to Boarding the Aircraft / Gate ... Just to have a Crew Member tell tem they have to check their bag / item cause it is deemed oversized (or too much Carry On for 1 Pax). That is when things really suck, cause you can be sure chances are higher that lone item ain't gonna make the flight (late check in) or get lost.
Play it safe... Play by the rules, and plan your carry on / check in items ahead.
Acceptable sizes (and items) can be found listed on each individual airline website... As well if there is a charge for Carry On Items*
* Charging per item is the latest thing / money grab with the deep discount airlines.