256 x 256 for the image if its stopping on the upload image
Have you tried as your testing your upload, deleting from your steam workshop all the tests so your fresh and then cook and upload fill in all the title description etc
What I do
Check your world settings enable world composition (loads all your orange leves)
Save all
Cook total conversion
0 errors 660 warnings
then upload
upload takes about 10 minutes steams slow and my map is about 256mb but its sometimes says its bigger like 1gig ? no idea
Then to check it log back into steam subscribe to the mod download what ever it gives you start ark now it will download the rest of the mod and put it in your directory.
Now you can select all the mod options eg total conversion bottom right and host a game if that works its pretty much well working 
Hope that helps