I had the same issue with map size and I found a complicated, but successful solution.
I downloaded a program called SqliteBrowser (
http://sqlitebrowser.org/)
Using the program I navigated to Documents\My Games\Sid Meier's Civilization 5\ModUserData\GTAS_AdvancedSetupMod-14.db
(You should of course make a backup of the file first. )
Once it was opened I went to the Browse data tab and selected MapData
Clicking on the data cell (it begins with 1***5 in my case) it opens up the edit window.
the data is in one long string but by changing the second digit after the word worldSize to a 5 I was able to play on a huge map.
so i started with
...worldSize***3***3***....
and changed it to
...worldSize***3***5***....
this seemed to change the default map size to huge after I saved the file.
My guess is that
1 = Tiny
2 = Small
3 = Standard
4 = Large
5 = Huge
It might also be possible to change the Map Type this way but I have not tried this yet.
I know this is not a simple solution, but it seems to work.