I am not sure what you mean with box/circle hull - use either a box hull, or a sphere hull. The size also does matter. Experiment with it. You have to live with the ODE physics as we aren't going to fix or improve ODE, so you must tweak the parameters when you get physics problems. Alternatively, you can use the PhysX plugin by Chris3D. It uses the same functions as the native physics, so the conversion should be pretty simple.