Honestly the easiest way to make the maps bigger, while still keeping within the 32bit restrictions (for those nubs among us like myself), is to simply have..... *drumroll*
LESS FRIGGIN USELESS WATER!!!!! 
I mean come on, in what way are the water tiles (often like 40-50% of the entire map!) useful? In NO way what so ever..
Okay so, they add landscape and variety, and a reason for making boats.
But come on, why not have say 4 water tiles in width absolute MAX, and have continents or islands between? Is there ANY reason to have up to 20x20 water tiles forming a huge open void that cant be used in anyway what so ever in gameplay terms?? No..
So my vote is to simply make the default seed maps have less water, 20% at max (absolute serious ultra-max). And never huge oceans, just surround the edges of the map with water, and basically have small lakes or wide rivers (no, not the river system, but done by the normal water brush) through the entire map to seperate continents, no more than 2-4 tiles in width. 
That way, we'd still have variety in terms of map design, we'd still have a use for boats to cross between continents / islands, but we'd also have MORE gameplay space for the same memory usage, and MORE open sprawling landscapes in the same map sizes, MORE space to found cities, MORE resource locations (could help prevent 5 resources clustered up if they have more space to spread out?), more everything..
Really, do this, now. Or i'll just make my own seed maps and delete the vanilla ones (can you?)
I have spoken! 
