Already The Sims 2 is very nearly unlimited in size, or at least exceeds what any sane person would be able to use in a lifetime. The three neighborhoods that come with the game have internal names N001 to N003, and the first you make yourself becomes N004. So you can have up to 999 neighborhoods, hardware permitting. If you have either of the expansion packs NightLife or Open for Business, you can add sub-neighborhoods. These share population with the main neighborhood, but have their own maps. These are named N001_Suburb001 etc, so you can have 999 suburbs in each of your 999 neighborhoods. In addition you can have 999 university campuses and 999 vacation resorts. Each map can accommodate a few hundred sims at least. So, hardware permitting, I could have a sim population larger than the United States. Obviously that would take a few lifetimes.
This is really nice and I appreciate it, but doesn't it seem... excessively massive? Or more like excessively excessive. It's not like any of us is going to play any of these games for several hundred years, at least not without some really stunning medical breakthroughs!