The generally accepted rule for coturnix is 1 square foot of of floor space per bird, and more is better. The height is debatable, but it is my experience, that coturnix generate more inertia from their legs by jumping, than they generate by their wings, so any height pen is fine, but since they are ground dwellers, they will generally stay on the bottom of any height pen. They do not usually "perch" or roost the same way most birds and even most galiformes roost.
Yes they can fly! Some more/better than others, but if they can run away! They usually take that route.
All my quail are housed on wire, no less than 30 inches off the ground. Most people that raise quail/coturnix quail, don't let them contact the ground. This is for anti-parasite reasons. Both internal and external.
Pen design is a matter of preference, but I go for network standard dimensions. The wire I use is either 24 or 36 inches wide, so my pen dimensions follow that rule. Lumber is usually 8 feet long, so I live within that rule.
I would never build a pen that is 30 inches by 9 feet, because the waste of labor/materials to construct such an oddball pen is a waste of time and materials.
I would suggest, at least a 2'X2' pen at any height. Better yet, a 2'x4' pen, because you may want more of the suckers in the future
