I remember the time when my brother in law first got a portable toilet. I think it was a porta potti brand but I cant remember excatly. But we stopped at the last roadside rest area that had a dump point before home to drop the contents - he got a little enthusiastic with the dump speed and managed to splash shit all over himself. Of course we all thought it was hilarous except for my BIL.
Anyway when we brought ours I just found one with the biggest crap tank I could - I think its 15 or 20L