Questions Farpoint 70

I just received the Osprey Farpoint 70. When I purchased it I didn't see any size option (S,M...), is there only one size for this model?

I bought the 70L because the 55L is sold out at all the stores in my area. Is it too big for a 12 day trip or is it the right size?

The back pack comes without a raincover. I would like to use the raincover to protect the backpack in the airplane. Which rain cover or protection do you recommend?

Thank you very much!