| Size is very inheritable; perhaps as much as 85% of the parental size will pass on to the next generation. You mentioned the koi being stunted which prevents an assessment of their size and therefore the known potential of size that is genetically inheritable. Breeding siblings together will also have some significant genetic drawbacks… size will suffer along with general vigor. Only inbreed when you need to fix a genetic trait, and only then.
I have some wonderful three year old koi selected for future breeding. My personal discipline is not to use them until they are wonderful 5-6 year old koi. It’s at this age when their true genetic qualities are proven viable for breeding. That’s the best advice I can give you. Let the koi themselves speak to you. Don’t speak for the koi.
Mark |