You didn't have her sit on the other sizes???? I think it's only something you can see and judge for yourself. She will need room to grow but only you can judge when her last growth spurt was or if she is genetically gifted with long or short legs. K is 7 and her 20" BMX bike is a bit big for her. She is finally growing into it just this past month or so, this summer it was too big but she rode it anyway and occasionally she made it around with out crashing...LOL.
If money is tight and this will be lasting her for at least a year then go with the larger size but if it's too tall then she won't be able to ride it until much later with the training wheels removed. She might be interested later this year in doing that. At BMX we have several "just turned 4 years old" kids racing. One is rated as Expert girl, at 4 years old. She travels all over and races and started at 3.
J is 40" by 40 lbs and he rides the small one from Wal-Mart with training wheels. He will be able to go to the bigger size by Summer I think. He is really slimming down and shooting up. He looks like a preschooler now instead of a toddler :(
When we go for his first BMX bike it will be either the mini or the regular 20". It depends on if he gets legs long enough or if the stay short like his dads. They can be expensive so I hope we can find a good used one when the time comes for him to start racing.