I don't have an actual x-pen, I use NIC grids. The pens I have are 2 girds high which is 28". None of my rabbits try to get out. I would not go shorter than that as I know my rabbits will get over 1 grid (14 inches) easily.
There are rabbits that can and will get over 28", so need a taller pen. X-pens tend to come in heights that go up by 6", so you get 18, 24, 30, 36, 42 and 48 inches.
Most people seem to use 36" and taller ones.
Flemish giants are big rabbits, so could reach the top of a pen if they stand up fully. This means you may need a taller pen or make a top for it.