Tall maps are possible after the patch with the New World scenario in it. Prior to that, no Firaxis maps for Civ5 had attempted to be taller than wide, and some bugs were around, undiscovered. You could go back to the tall-map idea now. (The world builder still caps at 80-tall, but the game engine now supports tall maps -- as long as, again, the map as a whole does not overwhelm system and video memory.)
Graphics being good is important for every video game. No 32-bit game is ever again going to support maps as large as Civ4, much less Civ3. Civ5, however, is not really a 32-bit game -- not intrinsically. And Steam is going to have to make the leap to 64bit support some time; it's only a question of when.
However, Civ5 is also a 1UPT game, and moving large armies across a mega map one unit at a time would seem to be less fun than moving Stacks of Doom in the past.
I know from personal experience running the Realms Beyond Epics in Civ3, we only ever ran one event on a Huge map. The game was so long, few participants finished, and I personally never was able to complete my report for my entry. I spent weeks on it and still wasn't half done. ... At some point, even fans of large and huge maps (like myself!) have to confront the reality that there can be such a thing as too much. We held a myriad of Civ3 events on maps from Small to Large, but only a couple on Tiny and one (ever) on Huge -- and those latter seemed better on paper than they worked out in practice. Small, Standard, and Large really are the sweet spot for the franchise.
Civ5 can handle Huge (if barely -- within the 32bit constraints) and a lot of work went in to optimizations to make it so. But Terra is oversized -- a necessary evil for its concept to work out in practice. Terra works great in Civ5, in the Small to Large window. Huge Terra really wants 64-bit to run properly, as will any other script needing to bust out past the 32-bit memory boundary.
Civ5 on Large is what I like to play, and what I have played the most. I would expect your script to play best at this size: large enough to enjoy all the things unique to the script, but not too large to where 1UPT mechanics start to drag. As much as I have always been a fan of the largest maps possible, in review, my best games on average came on Large, not Huge. I'm sure that's not true for everyone; it is just my personal observation and perspective.
- Sirian