Is your model scaled at all ? check that you are setting the object up and allowing it to use scaling..
Otherwise there is a command in there for setting/changing the radius of the sphere collision I think.
Lastly, when you say "biped" what are you referring to ? Do you mean simply a single object model that is a humanoid(2legs 2arms, thus biped) or are you referring to the 3Dmax BIPED animation controller ?
either way, the reason you could have only a small sphere is that there are multiple objects or limbs making up your overall biped object and that only one is being setup for collision and is the only one that has the sphere around it.... Im not sure if there are commands the check for limbs or to include multiple limbs or not im sorry, but looking through the docs for it should help.
If it ain't broke.... DONT FIX IT !!!