Breastmilk will expand approximately 10% when it freezes. You will be safe if you leave about 1/2oz of air in the pouch. This does NOT need to be a precise measurement!
When pouches are frozen flat, they have an increased surface area (for a given volume of milk), which translates to faster freezing, and faster thawing.
The less time your breastmilk spends at room temperature, the better!