most people will mod a house or a town, add a quest or new armour. But is is possible to make an expansion? add new content, new story, new quests, new places, new dungeons?

How far does 150mb's take you in this? what are some popular mods like this to look off of by example? and if you go over 150mb's is there other trusted sites that people also look too for mods, aside from steams workshop?

The actual data itself doesn't take up anything at all. It's the external resources, textures, meshes and such which really add to the size. And the Skyrim Nexus is pretty much the de-facto standard for uploading mods, even more so than the workshop as it allows much more control and doesn't have as many arbitrary limits.

Ok cause I was testing out the mod creation kit and I have an idea for an expansion, but as I built my first dungeon I noticed that as I got half way, my size was 5mb's out of 150 and I was like.... Oo. So either I was doing something wrong, or......

What is the communities thoughts or "standards" on size in terms of mod size - if I was to carry out my plans to build this new content?

If a simple dungeon's ESP file was 5MB then you did something wrong.

Dawnguard's plugin file is only 24MB. Hearthfire is only 3MB. The core Skyrim.esm file is only 243MB.

Resources will be big, yes (like the BSA), but unless you're adding new meshes/textures you won't need to worry about that.

hmmm. Ok thank you

That number in your dungeon (if you are looking in the viewer) is the memory required for your particular cell, notice if you jump around the various cells, it will change the # MB/157 MB, due to the usage of the cell. Take a look at your .esp and other files (if any) to determine the actual file size.

Yeah the biggest space-takers are LOD, and a lot of assets. Falskaar currently has a few minor new assets, a LOT of voice lines, and LOD for a 2x2 quad worldspace, and it's about 90mb packed into a BSA. So you can do quite a lot with 150mb if you are 'wise' about how it is 'spent'. If you add a whole bunch of high res textures, you'll use it up quickly. If you add a bunch of new dungeons in an esp, you could remake half of Skyrim.