5'3" to 6' is a huge range.
You might look at the Burley LWB. These are available at entry level prices (stating with the Sand Point: $749) and may accomodate the range you need. She would be able to reach the ground, the the larger rear wheel would mean the weight distribution would be less compromised when you were on the bike.
If you test one of these, make sure the seat back is tilted all the way up and the seat pan is all the way back. There are a lot of adjustments on the seat; some of them are tedious and the dealer may not have messed with them all. Lots of adjustments available for the bars as well, so if they don't feel right, tinker.
Some people get bleacher butt from the seat base on this bike, but it might not be a big issue for either of you once you get used to it.