Easycore, don't worry, i usually start working with 1024 textures and later resize them, so in the end the file size would be much smaller than 20 mbs, talk about 40 mbs;)
It will be fps wise, because cities are not large in that area.Nor will i add too much clutter.
Raptor, i have a few working ma terrains that are not submitted (well kinda working - no vspawns, sbs or such small things).
I have also made maps of this size and detail before, i pretty much know how stuff works( or at least i think so;) )