Flow Frames have a 5.8mm cell width.
We experimented with a range of cell sizes and measured a lot of natural comb.
When bees make cells specifically to store honey, they often make much larger cells (as compared to making cells for brood). As Flow Frames are designed for honey only, we chose a larger cell size. We also wanted the queen to be less interested in laying in the frames for those who experiment with no queen excluder (although we do encourage the use of a queen excluder).
The size we chose is too large for worker brood and just smaller than drone brood.
We supply and encourage the use of foundationless brood frames to allow the bees to create the perfect sized comb to suit their colony’s needs in their brood nest.