It's the number of polygons and the size of the textures that matter.
A car of 30,000 polygons, which is fine for a standard model for animation and rendering, would grind DarkBASIC to a halt.
You should be trying to keep all of your models below 1,000 polys, with the less significant items amking up the scene even lower. In a scene with one character and very little else, you could push the poly count up to 2000 - 3000, for a cut-scene for example.
Textures should also be 1024x1024 at the most for the main character/objects, but perhaps 256x256 for less significant items again.
These are just guidelines. You can be flexible, as long as you consider the overall load on your game at any given time.
And 3DS is no longer supported in DarkBASIC. You should use X models, or better still convert to DB Pro's DBO format.