What will she use it for? Smaller wheels accelerate faster but they also have less mass so you have to keep pedaling. Smaller wheels tend to be stronger than larger wheels and lighter . They also allow the bike to be folded smaller. 20 inch wheels are a good size. My Dahon piccolo with 16 inch wheels wieghs 26 lbs.I also own a Bike friday metro a similiar one from BF costs around 700 dollars . I really like riding my BF but I hate folding it . It is a 7 speed with an 86 inch top and 36 inch low. Dahons speed P8 comes well recommended and It does not drive me bonkers to fold a Dahon.