
The thing is no-one has to breed a litter, especially in a breed where there already exists a sizable rescue problem. So why does the OP want to breed if they are not showing. No mention is made that the dogs are working dogs etc.
The purpose of breeding is to maintain and improve the breed, and this is generally done by proving ones stock in the ring or working field, and then if good enough to consider breeding if one has a long term goal in mind.
The breed although exploding in numbers is yet still a fledgling breed in this country so there are few breeders with a long history in the breed which is needed to help newer breeders research into what lies behind their dogs. Few UK breeders for example will have known/seen every dog in a 5 generation pedigree. when I started I could approach breeders for information on all the dogs in my bitches background and ditto the prospective sires.
Now just 16 years on I know a lot of the dogs in my and other pedigrees personally, can remember their characters and those of their offspring, the faults and virtues they produced as well as some of the minor/subtle points of type they passed on for generations. then of course any health issues can be tracked too, and best steps to minimise their likelihood, without throwing out the baby with the bathwater.
As for your particular bitch, the breed has a range of size and she is outside of that, where you generally would be looking for her to be about mid range. Also it is often the case that the more compact animals are sounder and typier than the bigger ones, so judges will and should favour them.
Generally if you mate a large and small individual you will get large and small pups, rarely the middle of the size range your after.
If you choose a medium size dog who has medium size ancestors then you will get some like him, and some oversize like yours.
When breeding you need to have a long range plan. You need to have a look at your stock and the breed as a whole to know what ingredients you have available for your recipe, and when making breeding choices your thinking of at least the next generation beyond that. In other words what you are likely to do with the offspring, what will be likely available when you come to breed from them, and where they will fit into the breeds gene pool.