Did you read the "Installation" section of the OP?
It gets downloaded to <Shogun installation>/modding. If you select "Play Game" on the Tools in Steam, it will open that folder in Windows Explorer so you can unzip it.
It may turn out the 100MB is a hard limit set by the Workshop (it's the same for Skyrim IIUC), we'll have to work around this - probably by splitting mods into several Workshop items that won't work individually. Or split them up in a way that they will and be extensions of each other.
I'm sure I can get a "Split Mod Pack" feature going to do this automatically.
Or modders need to split up their mod on their own, into individual parts by function: 1. split up additional units into different packs 2. another one containing GUI stuff (like buttons, loading screens, etc) 3. Localization packages (I haven't created large enough mods to be able to tell if that can be done at all, and if so how). That's obviously a lot of work, but again I'm sure I can provide tool support if people tell me what's needed.