Enter dimensions (mm/cm/doesn't matter, just be consistent) for three dimensions, and hit "add sphere". You can do this multiple times
Models will appear in a line. If you refresh the page things will reset.
Models are always a spheroid which is stretched to match the x,y,z coordinates given. Obviously it won't be perfectly accurate but should give an idea of scale I hope.
You can rotate the camera with left mouse, move it with right, and zoom with middle. Mobile controls may vary.