Depends on the carrier as many have different limits. Air Asia, for instance, regulates both the size of the bag and the weight of the bag and its contents. The metric physical dimensions on its website roughly equate to the 22" x 14" x 9" that is standard on US domestic carriers. Those set of dimensions produce 2772 cubic inches or approximately 45 liters.

You've got to watch out for the weight requirement, however. Air Asia says 7 kilograms, and that generally includes the weight of the bag. Seven kg equals a little over 15 pounds. I've found it easy to stuff over 20 pounds into a 45L bag.

And a third consideration: although some folks talk in total linear inches or centimeters, airlines really set a maximum for each of three dimensions. Thus, a bag that might total 45 linear inches might violate the Air Asia rules by being 25" x 12" x 8"--equal to the max cumulative linear inches and less than the max in two dimensions but over on the third.

To make it even more interesting, your bag will have to be small enough to fit the airline with the smallest limits. Tiger has max dimensions of 21" x 15" x 9". And a very confusing statement that seems to imply that the total weight of a cabin bag, a personal item, and its contents must not exceed 10kg (22lbs). So, if you fly both Tiger and Air Asia, you intend to fly with a fully packed bag on each airline, and they both strictly enforce their limits, your packed bag cannot exceed more than 15 pounds and 21" x 14" x 9".

Confusing? Yes. Don't try to straighten out this confusion in the middle of your trip when you're at an airport trying to figure out which of two probably essential items you may need to dump in order to get under the limits of the airline you're about to fly that day.