Please consider your mares size,
bone structure and mental temperament,
IE. Quite, Fizzy, Lazy etc etc.
The same should also apply to the sire of choice to gain required breeding excellence
in resulting progeny's physical and mental conformation.
So in simple terms try to avoid mating your mare with a sire who has the same body shape,bone structure and mental temperament as your mare.
Also take into account your broodmares full bloodlines. not just the sire of your mare.
The broodmares dam and grand dam bloodlines are just as important when breeding to gain the best progeny outcome for your broodmare.