Technically, one that fits in the 'frame' available for measure at most airports. But, of course, that doesn’t help until you get TO the airport, when it may be too late.
Stand the suitcase upright.
Measure it from the floor to the top of the handle (not the extended handle for wheeling, if it has one, but the ‘regular’ handle for carrying).
Then measure from left to right, again including any handle/s.
Then measure from front to back (or lay it down and measure from floor up again) including any pockets.
Add the three numbers together.
Check your airline’s website, but generally the three numbers can’t total any more than 42 inches.
Oh, and you each get one carry-on and one 'personal item' such as a handbag, briefcase, computer case, etc. That item has to fit completely underthe seat in front of you.