Nobody has ever said it should be a size, but you want to be able to get your toes under- 4-5”? and then that bottom rail is structural so it’s probably 3-4”, then a sheet of ply on top and you aren’t far off 9”. Ian
Its a trade off you can either have some storage below the bottom rail, as you have with less room available above, which unless it is fully enclosed just fills with dust and shavings. Or just enough for your toes. Use your 9" for deep storage with items in plastic boxes, about 6" in for toe room.