Did you create those primitive shapes (cone, sphere, box) with the D3DX routines to make them LPD3DXMESH? If so, can you just use the parameters you fed in there for width, radius, height, etc and use them for Bullet?
btw ... getting the radius of a mesh is simply finding the min/max vertices and constructing a bounding sphere (same with the box). So that gives you the box and sphere. The same technique would give you the radius of the cone around the y-axis and the bounding box extent gives you the height.