I'm using XXL World by nevermind:
http://forums.civfanatics.com/showthread.php?t=183746
However, my version of this mod is highly customized, and I'm not even sure if it works correctly with BtS 3.19. I do recall that I updated lots of map scripts which the original mod missed.
Depending on your hardware, your personal tolerance of inter-turn wait time, and the amount of other resource-intensive mods you're running, I suggest to increase map size gradually and see how it works. Otherwise you run the risk of slow gameplay and/or MAF crashes.
Personally, I completed several games on XXL size worlds (40x25 tiles, i.e. 150% the size of "huge") with 34 civs on them, no other mods running except BUG and BetterAI. I usually get a noticeable (but bearable) slowdown once the AIs have Astronomy. Somewhat later I start running into MAF crashes when I alt-tabbed out of the fullscreen game and then switch back to it. When this happens, I switch to windowed mode, which so far always solved the problem. I also have two distinct user profiles, one for the early game with all eye candy turned on, and one for the late game (with everything set to enhance performance). The biggest time-eater in the inter-turn phase is the option to see friendly or enemy units move, so I switch that off in the late game, and check the message log for warnings on approaching enemies.
My machine is an Athlon64 3500+ (single-core), 2GB RAM, GeForce 7600 GT 256 MB, Windows XP SP3. So it's pretty weak by today's gaming standards, yet I can and do enjoy Civ4 games on super-huge maps.