How to choose the bead size of a Panchamukhi Mala (varies between 5 mm to 8 mm)?

They are of the same quality; it is just that it's a question of convenience. The smaller ones cost more simply because the rudraksha is not plucked from a tree, they wait for it to fall down, fully ripe and fall down. So, it is much harder to pick up the smaller beads in the mountain so they cost more; otherwise they're about the same.