Is there any difference in the bag sizes? 

YES.  The 12oz (green) bags and the 5lb bags are both nitrogen-flushed and heat-sealed and have a one-way degassing valve.  This process keeps the oxygen (coffee enemy) out of the bag for extended freshness.  The valve allows for the coffee to off-gas after the roast and gently escape without building pressure inside.  Since it is one-way, it does not allow oxygen back in the bag.  The 1lb bags are NOT heat-sealed or nitrogen flushed.  If you are ordering 1lb bags, we recommend you drink them within 2 weeks of the roast date.  

The 12oz and 5lb bags are not EZ Peel, so they must be cut open.  The 1lb bags are simply rolled closed.  5lb bags come whole bean only.  

If you were going to give Beanstock Coffee as a gift, which bag would you recommend?  

Definitely the 12oz bags.  They look better, stay fresh longer and hold up better with handling.  

Where are the roast dates on the bags?

The 1lb and 5lb bags have them stamped on the back.  The 12oz bags have them underneath on the bottom of the bag.