Teryc
You don't say how much under 5 foot you are. I think all K21's have adjustable rudder pedals. Most have optionally fittable adjustable back rests which will move the back of the seat forward by about 6 inches. With these adjustments and wearing a parachute, I think a K21 should be comfortable for most 5 foot tall people although you may need to sit on a suitable cushion to raise your head height a bit. Certainly, at my club, we instruct plenty of people around 5 foot tall without needing much in the way of additional cushions. If you do need more cushions, clubs should have energy absorbing cushions available for use in their two seaters which they will regularly use when instructing smaller students.
If you are very light, and especially if flying with a light instructor, you may need additional ballast. K21s are equipped with mounts for installing fixed ballast weights - ballast pads under the seat cushion should not be required.
The other commonly used instructional glider is the K13. At my club we have wooden seat backs to use in the K13s for smaller pupils which again move the effective seat back forward about 6 inches.
Once you move on to single seaters, my experience is that being too tall is more often a problem than being too short!