You could make the map be a totally open arena but with fence walls blocking your path according to the layout... but that would be crap and the judges probably wouldnt like it. Remember you have freedom to use vertical gameplay to make the map bigger.
I'd like to add one more thing, since Habboi's question is actually an interesting one:
outdoors or indoors?There are exceptions, but maps that can be strictely sorted into one of these two categories tend to be worse than maps that combine both.
In deathmatch, the goal is for players to have fun: Now that means that
everyone needs to enjoy the map. From our shotgun-lovin friends, fans the RPG, the snark protectors, the tau-cannon devils, crossbow snipers, crowbar killers... etc
If you build your map with most of the weapons (both in hldm and hl2dm) in mind, you will see that different type of areas are good for some weapons and bad for others: indoor hallways are good for close combat shotgun action, dark spots are ideal to place your deadly slam charges, open areas are a great place to test your RPG or crossbow skills, etc.
My point is: find the balance between indoor and outdoor, and consider that entity placement is almost half of what will make your map be what it is. A good combination of these things will prevent your map from getting old too soon.
Remember, some people can feel claustrophobia in games, others are not so good at long-range-aiming, but everyone needs a happy fragging time!!
sorry for the long post... it might also give you guys a hint on why I dont usually rate AIM type maps and such