poor wee lamb, lucky for her you are keen to give her a chance! Mine were all orphans mostly ones that would not feed properly for up to a week so would have been dead had i not persevered. Their start was extremely weak. They survived and are pets but I have the largest sheep in my area, they are healthy and strong, and some of them took a couple years to grow, as i said, but even the 'gimmer' (runt of triplets?) is only slightly shorter but strong and stocky. If you are willing to give her time, she may grow.
Why not give her supplementary feed? It will give her more of a chance to catch up and be a strong ewe for breeding wont it? Then if she is as strong as she can be healthwise her slow growth rate wont particularly be a factor in her offspring will it? I dont think it is always hereditory- i may be wrong. But i understand your reasoning for saying only keep the best for breeding. My two massive sheep came from quite smaller to medium sized ewes, and 15of the others came from miserable smaller sheep on very hard dryland with very little grass. I believe alot of their growth was to do with the good quality feed provided in the first two years.