So as I understand it the max size of a map can be 100MB or something. But how big of a map can you get into that 100MB? Does anyone know some huge map on the workshop I could look at to get a grasp on the limit? Or some other reference or something?

And also, what would be the best way to lower the file size of a map?

Before you hit the 100 MB limit, you will probably run into other limits first. One is the 1750 entity limit. The other is the map may be no more than 25 by 25 "blocks" in width or height. You can check my map, Baboona, to see what a map with 1634 entities in it looks like; it's also 24 by 18 blocks in dimension for all the rooms. But its only about 10 MBs in size.

EDIT: I assumed, perhaps incorrectly, that you were referring to in-game editor maps, but perhaps you were referring to workshop maps in general. If so, disregard.

This thread is a good place to see how to lower the size of a bsp.

As for the actual size of a 100MB bsp that's a bit hard to answer. The largest single bsp I can think of is probably the last level of HMW's "Try Anything Twice". You might want to test how size scales when adding levels together. Does size(level1) + size(level2) = size(level1 + level2)? If so then you could look at a bunch of maps (whose sizes add up to 100MB) to get a sense of scale.

Let's start off with some information about two maps I made:

Map 1:
Size: roughly 4000x4000x4000
1060 solids,
6355 faces,
930 point entities,
119 solid entities,
100 unique textures
Optimization: all in a big box, loads of lines of sight.
Size: 60 Mb

Map 2:
Size: roughly 9000x5000x3000
738 solids,
4473 faces,
1353 point entities,
255 solid entities,
85 unique textures
Optimization: three big rooms, with corridors, breaking lines of sight.
Size: 30 mb

As you can see, map 1 has about 40% more brushes.
Map 2 has about 45% more point entities and more then 100% more solid entities.
Still, map 2 is half the size of map 1.

The lightmaps in both maps are still 16 everywhere, and both are full compiles.
The 40% more brushes themselves do not explain the fact that the map is double the size.
Apparently though, the 40% more brushes influence the map size more then the 45% more point entities and the 100% more solid entities.

Conclusion:
The biggest drain on your map size are your brushes.

Why are brushes the biggest fillers of your file size?
Because they all have light maps.

So, in order to get your file's size down, while keeping the map big will be reducing the light maps.

1: Don't do a full compile. Full compile is LDR + HDR lightmap, so basically twice the lightmap size. That's quite a filler for your bsp size
2: Optimize, proper optimization will make light maps easier, thus also containing less information thus being smaller in file size.

The final, obvious point: use the default content, a custom 512x512 texture is already about 200kb, five custom textures (say for a skybox or something) will already take up 1% of your file size. custom sounds and models will probably take even more.

As for just technical stuff about limits etc:

https://developer.valvesoftware.com/wiki/Entity_limit

edit:

Quote:
As for the actual size of a 100MB bsp that's a bit hard to answer. The largest single bsp I can think of is probably the last level of HMW's "Try Anything Twice". You might want to test how size scales when adding levels together. Does size(level1) + size(level2) = size(level1 + level2)? If so then you could look at a bunch of maps (whose sizes add up to 100MB) to get a sense of scale.

Well, this highly depends on how you connect them: if you just connect them with a small corridor that also breaks visleaves it will be about the same, however, if you connect them with a nice view on each other, your visleaves get more complex and your file size won't be 1+1 = 2

Thanks guys! I did some testing and I hit the hammer limits way before I hit the 100mb.
I also seemed to be able to half the size by not going with the full compile.
So I guess this wont be a problem for me after all