IMO a magic wall would need ground on which to stand however, if the caster did cast 4 fire walls creating a box (1 north, 1 south, 1 east, 1 west) where the edges joined, I would allow the caster to cast a fifth fire wall "ceiling" thereby creating a box. BUT if a wall failed (say due to time expiration) I would rule that the ceiling would collapse passing over anyone inside the box doing the appropriate damage.
I may allow the "ceiling" to be supported by three walls but I'd have to think about it.
"Increase size (per additional 10'x10'x1' section) +3 PP"
What does this mean?
2 sections = 20' wide 10' high 1' thick only? Or 10' wide 10' high 2' thick? 10'wide 20' high 1' thick? Isn't it possible to use more magical power (PPts) to increase the height of the flames? (remember BBQ and grain alcohol oO)
I would allow the caster to determine where the additional section joined the original. So it could become either:
A 20" wide wall that is 10" high and 1' thick
A 10" wide wall that is 20" high and 1' thick
A 10" wide wall that is 10" high and 2' thick.
While 2' thick doesn't sound overly impressive, I would think that if a creature charged through it would be like going through 2 Fire Walls therefore 2 lots of damage would be rolled. Again that's just my opinion.