Omahan got the definition exactly right for clothing. The height tops out at 5"3", so Fjensen is right with under 5'4". Besides height it is usually implied that they are also slender/slim and smaller in stature. The only reason I know this is because my mother owned a womens clothing store and was a seamstress.
Alley I think the only thing I would add to a petite girl to make her a spinner would be the ability to spin (with or without basket)! That just makes my pants tingle at the thought!