Back in the olden days the maximum map size was 425 x 425, not the dinky 325 of today.
Would be nice to go back to it.
I loved those huge maps.
Did you have Enable Test Map Sizes Checked, in the options? There's no more Ludeoncris whatever it was called, Sizes except now it's called Extreme (test). There's 400x400 size bellow 350x350 only though.
I'm sure that 400x400 Map size sounds okay to me and to you as well...
Custom map size is what you are looking for.
https://ludeon.com/forums/index.php?topic=43498.0
But that mod didn't got updated from B18 yet.