Moshing

How big are mosh pits usually

Depends on the band.

As your user name is 'bmth' I'll mention bring me the horizon ;D

When I went to see them, their mosh pit took up the whole standing area. It was mad.
Same with when I went to see koRn.

But some bands aren't as big.

it mainly depends on the band and how good or how much the crowd loves them..or if the band themselves ask for it! whenever I went to go see aiden people started to mosh and stuff..and then htey asked us to make a big space between the two crowds and when they started to play to just crush into each other and mosh! so yea..it depends..