Our main studio mic/cue panels, which are 20" square, are mounted 19" from the floor at their lowset point, extending to 39" at their highest point.
Remember that, the higher you mount the panels, the longer each microphone (and other) cable will have to be, as usually the cable, given slack, will be pulled by gravity towards the floor, and extend to its destination in that repose.
The lower you mount you panels, the more you will have to bend down to plug something in.
Life is just one big trade-off.