That 8 megs is quite old limit, so I think it can be pushed little to around 16-20Mb. I recall that the Tempest pit already did pushed the texture & poly limits little, but it was one of the last pits from the official 3rd party development days (so it's quite new actually).
That 8Mb limit didn't include damage textures or night textures, so it should be enough to do a nice looking pit. It's not the size that matters, it's how you paint it.
Why we haven't seen a Gladiator pit repaint? Because it doesn't need one.