hi there
i am also making my custom map recently, and i think the 16M limitation is just ~16m, which means, 4097*4097 or 2049*8193 should do just fine.
i have maps of size 2049*8193 running flawlessly on my machine, some of them from the workshop, and i've made my own map like 2049*8193.
however i am not sure about 18M px maps.
hope that helps!
i have problem uploading to steam workshop though, cannot find proper button uploading, so if you finished your map and succeeds in doing so, please teach me how to do it!
cheers