I had a little trouble finding a couple of the spots I know are there (I placed them on chessy within the past year and pulled them up about 3 months ago to change shards). Here are some that I did find, including a couple of new ones by the fighting arena thing:
An 8x13 or a 9x11 fits here depending on preferences.
Another 8x13.
9x10
This is a 10x14 by the new fighting arena.
13x8 also by the new fighting arena.
This is a classic L-Shape. Can't upgrade it, but...
Once I re-locate the other two spots I can post pics of their locations as well. As some have said, there are lots of 7x7 and 8x7 sized spots all over. Some might could fit something a tile or two bigger, but I haven't looked for them.