I use a huge map I built just to test nukes. The only annoying thing about that map is that it's a bit crashy (and the fault aren't the nukes, it crashes if I just spawn some bots/pawns and start to gib them, the map will eventually crash as well), but that may be because it's just too big and perhaps because I added quite some max-radius lights in there.
As far as detail goes, the "dirty" trick with UT2k4 is distance fog. If you see past the fog, depending on your machine you may lag, plus UT2k4 static meshes are stored in the graphic card RAM, not system RAM, unlike BSP and meshes in UT1 for example (the same with the rendering, UT2k4: GPU, UT1: CPU makes the hard work, then sends the leftovers to the GPU, the GPU is quite useless in the process, unless you use high-res textures), so UT2k4 is much faster rendering a 10.000+ polys model than UT1 is rendering a 1.000+ model).
Also, Waffnuffy already proved that you can build a decent looking huge map in UT1 (I guess you know his project, it looks quite good imo), and I guess it doesn't crash since he doesn't go easy with the engine.