Also if you only give one measurement "NaN", it will do the same thing, but if a circle touches it it will get stuck in the top left corner. (Only tested this with a square)
In the level builders, if you type a letter for w/h, it will make the hight/width "NaN", and when you place it, it will have no collision and will freeze in place wherever you placed it.
Umm...I'm just going to say...maybe you should picked a character other than Mario...the only reason is this: http://www.youtube.com/watch?v=7WMSQNVhMqA and this: http://www.youtube.com/watch?v=jcxW_ciP5R4 . I mean, they are just theories but... still...