The problem is that if you are referring to Houx proper, not the annexe, they can vary in size...
I recall they are supposed to be 8x5m, but I saw some spots near trees etc that were bigger, and some thinner. I feel really sorry for people with motorhomes etc as you can get really stuck if you are unlucky as there is no flexibility to move as the spaces are numbered.
All that said, 80-90% are okay size wise - assuming you have spaces together and not split between rows (how can you tell if there is a "break" in your ticket numbers on the field?!