There are a few .X exporters for 3ds max. I'm not sure what release of 3DS you are using.. but Effectware has one (that works crappy).. Panda has one that works well (but the brushes are such that the DBP BSP Converter chokes hard on it)..
But I'm certain there's probably a way to convert a 3DS .X file into a csg that'll allow for BSP conversion.
Hmm.. I just use Ken's .X code that indi reference (for obvious reasons).. keep your single model under about 1500 polys or it will slow down. Alternatively, go to your EditMesh modifier and use the "Detach" feature to create several objects from one. This way DBP's automatic "on-screen" code makes your FPS go up dramatically and you can make it almost any size.
Loki D'Cat - Modeller, Composer, Animator
Nerdsoft Creations
Machine Specs: Athlon XP1800+, ATi Radeon 8500 64mb, 1GB RAM, Windows 2000 AdvSvr