The stepsize is only relevant when collision is ON. It is the maximum height difference allowed when moving through a world. For example, if stepsize is 0.3 and the height of the ground in front of you is 0.4 then you won't be able to move forward. If the height of the ground were below 0.3 then you would be able to move forward and Vizard would automatically put you the correct eyeheight above the ground.