I don't know my friend,the current grid can't be filled completely anyway,especially if you use lightmapping(on 32 bit systems that is,since there's a 2GB memory cap).
I tried making a town before,and ended up creating 3/4ths of it successfuly with lightmapping and rain effects and it ran at 28-30 fps,which considering my system is pretty decent,is not a good sign.
You could however use some trickery.I'll give you a few examples :
1.I remember some old games,in which when you go say 2 blocks down,another level loads,showing the other buildings and basically that reduces resource usage and provides a nifty workaround for the grid limit.With a mod like RPG Mod,you can go back to the previous level (a matter of a simple fpi script placed in a triggerzone) and divide your town into 4 areas or whatever suits your needs(but if it's a multiplayer arena you're building,then you can't do that).
It could be annoying for some and no,it's not ideal,but it would provide a better atmosphere than out-of-scale buildings and short streets.
2.You could use billboards for the town outskirts,rather than actual building models.Saves space.
3.If you just happen to be able to fit your town in the current grid , but you have lightmapping problems,make some of the building dynamic.
4.You could mod the source,however I don't know how that'll turn out.I'm aware that the X10 version of FPSC has a mod which makes the grid 2 times bigger,but it's not perfect there either.
These are a few ideas off-top,but I encourage you to try them.
