Size does not matter. What matters is will the dog hunt in the style you want.

Each breed listed are good flushing dogs and will be good retrievers. The Boykin breed is better known for its waterfowl retrieves more so than the Springer. The Springer is considered more of an "upland" dog but it also can be trained to water retrieve.

I have seen both in the field and have hunted behind a springer or two but o Boykins.
The con for me would be coat maintenance. That soft hair will stay wet for ever once wet and soft fur is a burr magnet. So they must be trimmed a bit to compensate for that.

If that is the breed you want - then find a breeder and visit their kennel as often as you can. And I would do a bit of internet searching to the pros and cons.

JW