For Standardbred the mare has to be of show quality, not coarse, good bone, strong correct quarters and look balanced overall, with some width to her. She needs to be a good mover. We don't worry if the shoulder, neck or head is a bit short/heavy provided it's in balance as this family is strong in putting on fronts. Size is not an issue so long as she is a good sort.
For Thoroughbreds it's similar requirements although they tend to be good fronted already, we more look to avoid a long back and a long neck, crooked legs.
Basically only because we know what this family produces and so what type of mare is needed to get a good result; nobody can afford to waste their time and money experimenting in this economy!
this sb mare is a good looking horse but not that solid built unfort she has a very bad injury from racing and is sore when running but when she was injury free she had a massive ex trot. the foal that she had last year is built like a little tank and is so pretty i was thinking to breed to this filly i wouldnt be able to for years to come