So I know that you can go over the 128 megabyte filesize limit for multiplayer maps using Tool++. I remember xxxheadlong was like 380 mb or something.

What's the maximum filesize limit for an SP map?
What about the max filesize limit for an MP map with Tool++

It's not Tool++ that lets you go over 128 mb for multiplayer maps, it's Tool_pro, which I think is the cause of my PC breaking down completely and needing a new hard drive. I was compiling a map at 128.64 mb and it was taking a very long time, then my PC froze up and the next time I turned it on it had to run chkdsk to try to fix the issue. I managed to backup all of my tags before my PC never worked again :/

So that's my story with Tool_pro (just got my PC back today completely wiped clean)

Oh. Well I think I will stick to the 128 mb limit then.

Yeah it's for an AI map that I'm thinking of making. I remember hearing that textures were the main killer of small filesize.

It's definitely most likely textures. For my test maps, almost half of it is textures.

And models shouldn't take up too much space. Unless they were highpoly everything.

Although it would be high-poly that would be the main cause of frame rate issues right?

I think halo renders a maximum of 50k tris so any extra faces do not show up when you are farther away, which is why you portal your maps. Correct me if I'm wrong.

Googling only seems to find me portal gun mods, are there any good portal tutorials out there?

sure the hek tutorial has a portal tutorial section in there, and there is one made by arkaman on you tube. http://www.youtube.com/watch?v=3SqygQGcPgU&feature=results_video&playnext=1&list=PL97A6E1AEB6A1D637
you can even just take the portals from another map and add them to your own map.
halo bsp tag converter>> http://hce.halomaps.org/index.cfm?fid=2029

get the model in max, select the portals, then save selected, open your map then just
merge the portals with your map in max, adjust to required size etc.

alright then, I will look into those tutorials