Don't make a DM arena for maps that don't need one... Ex- SG4, Holiday Resort, Wyverns Wake, etc. Maps that do need them, TSG1, TSG3, Breeze Island, etc.
Also another note, please make them relatively large... A small arena makes for awful PvP, having no chance to regenerate/run away. An example of this is Lobby Games ; It is almost impossible to fight a team on that tiny arena unless you get very lucky (Luckily, the one team I fought on there I beat).
So please, only make them on necessary maps, and please make them big.