Contemporary foundation is 5.4mm per cell, although some foundation is being produced at 4.9mm. Feral bees will make their worker cells as small as 4.7mm. While foundationless frames will allow bees to make cells the size they want, if they have been bred to be larger, then their initial comb will fit their current bigger size. After a few generations their cell size will eventually revert to their desired size. However, if your bees were originally bred in 5.4mm foundation, you may need to change out their brood comb to allow for new comb, which then may decrease over time.