My partner just picked up one of the new sea to submit compression dry bags. Pretty slick as they are waterproof but you can easily compress the air out. Could use them for all her spare clothes and get it into a pretty tight ball.
I stuff my puffies into their pocket or hood. The pocket is much tighter, better for packing in where volume is the issue. For day hunts stuffing into the hood works well, and also works for rain jackets.
I cant help you with what size a girls medium might require but I have my 5xl down puffy along with my down mittens stuffed in a sea to a 8 liter summit ultra sil dry bag. a medium will probably go into a 4 liter with room to spare for a hat and gloves. I keep my puffy inside the pack as that ultracsil is not going to hold up on the outside against briers and other hazards.