The nice thing about soccer too is that size isn't everything. Her losing the ball probably has less to do with her size than either her conditioning, maturity or her skill level. If she's recently traded up, she may still be growing into the role.
Also, being small doesn't limit her ability to build muscle, which will take time and special training/and condition like GT mentions here. It's the same way that not all of the tallest goalkeepers are the best and why there can be some (while few who meet the exact description I'm giving) very high level keepers who have made up for their height with technical ability and building strong powerful legs that can jump high (vertical), out (dive), and fast (reflexes). The same applies to any field player. Whether she plays up front, in the back, or in the mid, get her working on her technical skills first off. Make sure she has a good, positive position that allows her to play the ball comfortably. Get her passes to always be strong and on target. Work on vision (I wouldn't say that's as much technical as mental but still), ect.
But also get her physical ability up to par. Winning balls in the air, once again whether is is up front for a goal scoring opportunity, or at the back to prevents one, is extremely important. This starts with a solid core and building up her leg strength to give a her a good vertical. The core part is important because, especially if she is on the smaller end, no matter how high she can jump, she'll get knocked out the air. Of course, keeping in mind she is U13. If she subbing on for a player with speed, get your player some speed too! She can't be just technical at the ECNL level if/when/should she move up. Also, please please please don't forget upper body strength. If you have her working on her legs, it is so so so important that she is balanced. In addition to being balanced, having arm strength will add to speed and verticals if she has the proper technique, and give her strength to fight off an opposing player when trying to gain possession or win a 50/50 ball on the ground or in the air.
Having a solid physical ability on the field will only increase how good she is with the ball, and increase her ability and speed to learn technically.
Of course, she is U13, and please keep in mind that this kind of training takes a ton of commitment, and even then, it'll take time.