I'll try to chime in here although I have never sold assets on here, I have used some, hopefully what I can add to this conversation helps to some degree.

-when to consider an asset pack done?

This is something you need to figure out based on your own goals. It might be complete with just one sprite of a lantern. Might be a hard sell especially if you are charging for it, but if the only thing you wanted to sell is one sprite, one sprite is all it would be. Similarly, if it's just one character sprite, plus a spritesheet of actions the character can do (e.g. jumping running attacking etc) maybe thats all it is. Don't think just because you saw a 200+ piece medieval fantasy asset pack for sale, that you have to do that. Whatever your goal is, once you hit it, it's done.

-smallest reasonable size for an asset pack? 
(like is it ok to put out a pack that is basically just the same cat with a few different colors/textures applied)

Absolutely, the last asset pack I downloaded was a few tank models, in a few different colors each. If it's something specific, once you hit that goal it is ready to go.

-proper format across different styles mentioned above? 

Assuming you mean file format, the last asset pack I downloaded was all .png files (aside from the text file about licensing etc). You don't want to do any software specific file types because for example if you use photoshop, the guy who buys your asset pack may not have photoshop, now you are adding extra hoops for them to jump through to be able to use it. Vector files are great for any kind of asset that is likely to be edited after someone buys it, but I don't really know how common that is. Common file types like png are probably the easiest to be able to work with for most people.

-pricing?

Something reasonable. Free will always be the easiest way to get people to use it, if that is your goal. As much as I hate recommending ever working for free, if it's your first time doing something like this, consider making it free. Once you build a name and reputation charging makes more sense, by all means you can start out by charging but if you can't do free, it should probably be as cheap as makes sense in your head.

Just my own 2 pence or cents.

I'm fairly new to gamedev and asset creations, but I have uploaded some of my own assets in packs themed after my own game jam games.

With regards to the content, I will only use the assets I have created and therefore have full rights to use, so no headaches for copyright or licensing.

So far I have only uploaded them for free as the corresponding games are also free to play.

With regards to the structure of the packs, I always try to keep the files organised in relevant subdirectories for simplicity.

With regards to the file types, I try to be consistent.  For 2d images and sprites I use PNG, for audio, I try to use mp3, for 3D, I would include exports for the common file types and the BLEND files I used to create them.

If there is a lot of files to be included in the pack, it might be useful to include a contents list or readme that can help a user find particular assets more easily.

With regards to updates, this all depends on your own purpose for the pack.  If it is a standalone pack, I would feel it is complete when everything you want to make for it is in the pack.  If an update to this includes separate file formats, this could be done as a separate collection under the same store page.  If the update is more unique assets, I would either look at a DLC like model or, if there is a lot of new assets, a completely new Asset Pack.  One of the things I have realised is that there needs to be a defined cut off for when something is considered to be finished, otherwise you will never "finish" it.