Jenn, all you need now is practice. A good exercise to try is - As you're riding along start bringing your kite down from 11 o'clock, to just above the water. You will notice an increase in power as the kite descends. Straighten your front leg, push in with the back foot heel and feel the board edge bite. Pull in a little on the bar if you can. Push the bar out a bit and raise the kite slowly. Release the edge by standing tall and flattening the board. The power will go away. Repeat this simple exercise and you'll get used to the feel of the biting edge.
Incidentally, this is what you do to get pop to launch into a jump

.
Remember, you can't turn a kite if both back line are tight. You must move the bar out
a little to turn the kite.
Most importantly, relax and enjoy the learning experience.
Incidentally, back to your original query. I weigh 64kg and fly nothing bigger than a 10m Ozone Reo. On the Shinnster I can stay up wind in 12-15 knots. If you only weigh 50kg you'd need nothing bigger than a 9-10m kite, preferably 3 struts for lightness.