It's not the total number of entities in the map that causes that sz_getspace overflow problem, it's only when there's too many active entities visible (from the game's point of view) from your location. If you've been doing a fast vis or no vis at all (which means a lot more is "visible"), a full vis should fix it.
EDIT: forgot to add, it's mainly things that the game has to "think" a lot about, especially things that move or can be picked up and put down, including particle effects, weapons-fire, monsters, players, gibs, powerups, moving bmodels, lasers, that sort of thing. Things like lights, sounds and triggers are more for compiling and reference by the game and don't make any real difference.
Do you have a compiled bsp I can look at? It's happening because too much is going on in the same area. A few hint brushes might help if there's areas visible to each other that shouldn't be. It won't make any difference otherwise.
The bad news is, if it's happening right now, it's gonna get really bad when rhere's a bunch of players gibbing each other and firing weapons.