Well yes, but every size is important. Effect size allows you to create effects with different sizes(and positions). Size in Drawing attributes is good for live shows(e.g. you can control it with MIDI). Virtual device size is used to set maximal size, where the laser can draw(you don’t have to worry about hitting someone’s eye when playing with Drawing attributes size, when this size is properly set).
And finally the size in the USB Settings – this is size of USB hardware. There is a special converter in USB to allow you lower the size WITHOUT loosing resolution! So this size is good to lower when drawing on long distance.