For reasons you have already illuded to if you have a long body and stunt legs, the K21 isn't a brilliant fit - unless you are about the same thickness as a yellow pages as soon as you come to try and take full back stick (though if you're only 5ft tall i'm presuming not that heavy either, so chances are you need more foreward than backward movement) you'll find your midriff tends to get in the way.
Second the other suggestion of a K13. Have undertaken all sorts of BI flights with short people and we can normally get them fairly comfortable - they were designed before people got as big as they did.
It does beg the question of what single seaters you will fit into when solo - Ka6 should be OK (Flew one two weeks ago and that was tight) though it doesn't have an adjustable seat at all...Discus A should also be fine (Though may have the same problem as a K21).