Minimap Size: This can be done in-game (patched in sometime last year). Simply load up your game, go to Options, then Interface. You will see a slide-bar for Minimap Size.
Initial Steam Upload:
1. On your Steam client, go to Library --> Tools. Search for and download the Sid Meier's Civilization VI Development Tools (don't worry about downloading the large Development Assets for this)
2. Open up that up. The 3D splash screen has 3 buttons to click - the second one is Steam Workshop Uploader. Click that.
3. Hit "Begin."
4. Hit "Upload..."
5. Hit the "..." button to find and select the .modinfo file for PerfectWorld.
6. Hit "Next." On this screen you will select your workshop banner/preview image.
This image can only be selected or changed through the Workshop Uploader, not through Steam itself. Everything else on there - visibility, description, tags - can be changed at anytime on the Steam client or when you're on your browser logged into Steam. I always make sure to change visibility to Public on this screen, lest I forget and no one can see my mod!
7. Hit "Next" again - you can enter upload notes here but it's not necessary.
8. Hit "Upload."
From then on, you can go to your mod's page through the Steam client or your browser and add screenshots, change the title/description, etc. Note it usually takes a little bit of time - 30 minutes or so usually - before your mod actually pops up on the "most recent" tab, but anyone can actually see your mod with the link once you upload.
Note: Upload is very fast, but sometimes it will hang at Upload for unknown reasons. If it does, chances are your mod still uploaded. Go to the Steam workshop page for Civ 6 (https://steamcommunity.com/app/289070/workshop/), click the "Your Files" dropdown, then click "Files you've posted." If you see your mod there, it worked.
Updating on Steam
1. Open the Steam Workshop Uploader.
2. You'll see your mods in a list. Select the mod you want to update, on the right column you'll see "Upload a new version."
3. Click that - the rest of the steps are similar to the initial upload.
If the mod hangs while updating, again, chances are it still worked. To verify, go to your mod's page and check the "Updated" time stamp.