1024x1024, because older graphics cards freak out with tilesets larger than that, usually displaying everything as blank white. If you still need more tiles—though I don't know if this works in Alesan's Entities—try modifying portaltiles.png
in your custom graphics folder to expand them.
I should note that I got most of this info from the Wikia
, so make sure to check that out if you ever have a problem.