The "blow" test is when you open a new box of paint, try dropping a few balls down the insert. If they all drop through freely, the insert is too big. If they lodge somewhere in the insert, try blowing gently down it to free the ball. If the ball passes through without you going red in the face from blowing hard, it should be around the right size. Remember though to try a few balls not just one, as even in the same bag there can be some slight variation. Not too bad if you end up with an oversized insert that just means less accuracy & consistency, but a disaster if the insert is too small.
Try to get someone with a Freak to show you & remember - if in doubt go up a size rather than down.