I should also say.

You cant stop the bsp process splitting large blocks into many portals but you can help reduce the number of portals considerably also by making sure that blocks never actually overlap, which creates further splitting and many more portals. This can be painstaking by necessary if you want huge levels.

Do this by making sure all walls just touch each other using snap to grid or if possible never actually touch each other at all in places where you cant see the join, areas of darkness and so on. If under magnification (zoom) you just get them touching or aligning very closly but not touching and use high prec when you build your level the bsp processe will close the gap in many istances.

Unless you have bright lights behind minute cracks - in normal gameplay the player wont notice any that do remain - but you will save creating many extra portals this way.

Then you must ensure that you level is surrounded by a single hollow box (no gaps)or you will get uneccessary leak artifacts showing up even if the engine may not mind running the level.