1. Depends. For Octrees(fixed node), it's usually a set number of polygons per node. For KD Trees(balanced octrees), the same as octrees but the number of node depends on the number of polys on a scene. For BSP trees (a non-axis alingned KD tree), well, same as a KD tree but with addition of a thing called PVS (Polygon Visibility Set).
2. Collisions(assuming this is what you mean) is also dependent upon the spatial partitioning you use. For octrees, I just do a sphere to cube intersection for each node and if the sphere intersects a node, collide with all the polys inside the intersected node using the "plane equation".
3. I cannot help you there since I cannot make 3d models if my life depended on it. But basically just import (obj is the easiest) read the model and construct your octree, kd tree or bsp tree. You can save your tree on a file or calculate on the fly.
Tute (trees and quadtrees):http://rel.phatcode.net/mytutes/octreetute/octree_tute.zip
Binary and source for an octree:http://rel.phatcode.net/junk.php?id=88
PS. Are you sure BSP trees are what you need? Octrees and KD trees beat the crap out of BSPs in scenes that don't have lots of walls. ie. Outside scenes. BSP trees work best in dungeon type scenes.