In this case and to understand blitz collision system, could i consider a virtual sphere at the _center_ of the animated mesh with a radius of 0.1 ?
Yes, that'c right.
Next, this sphere seems to be used to check collisions with another mesh... Is is true ?
Yes, right again.
Could you set the sphere center position ? How blitz determine where is the mesh center ?
Yes again. The centre of the mesh is the mesh's pivot point. This is the point that the mesh is rotated around in rotations. Also, when you position a mesh at 10,0,10 ( just for example ) it's the pivot of your mesh which is placed at 10,0,10.
It's best to place your mesh's pivot in whatever you're using to model. The point you use as a pivot will vary according to the object you're using. In this instance, the object is a man, and you will most likely want the pivot to be right at this feet so that you can place him on the ground properly.
Also, be aware that you don't need to use a mesh for the collision source. You can just as easily create a pivot in blitz and set that as your collision object and give that a radius. Then you can load the mesh and parent the mesh to your pivot. This can save a lot of headaches further down the line, as I discovered to my cost a few times till someone ( I think it was Tigerz ) set my straight.