At a minimum plan for 10 sq ft per chicken (so 90 sq ft in your case) - however I highly suggest going over the minimum. The chickens will be happier and it makes life so much easier to avoid behavioral problems before they start, and more space gives you more options should you want to add more chickens in the future.
Traditional specs for the enclosed house is 4 square feet per bird......that is an established spec that goes back at least 100 years. That is probably double what most modern eye candy (which also tend to be poorly designed death traps) coops sold to unsuspecting buyers will suggest.
Rule of thumb (not an established spec that I have been able to tell) for an enclosed run is 10 SF per bird.
I had 500 square feet of pen for my first Flock of 5 Golden Comets.
My second Flock of 7 Barred Rocks were in a 100 square foot dog kennel. 14 square feet per chicken.
I bought a second dog kennel and added two sections to double the space. 28.5 square feet per chicken. They were much happier.
A month or so later I added the other two sections to triple the original space to nearly 43 square feet per chicken. About the same behavior as the first expansion.
So I would say 28.5 square feet per chicken is adequate. Bantams would probably be happy with less space.
I have the same breeds in that kennel. If you have a mixed flock with Bantams or aggressive breeds like Rhode Island Red, more space may be needed.
If you free range daily weather permitting. 14 square feet per chicken may be enough.
Make it at least 10 sq ft per chicken and also have roosts and things for them to hop up on. If you can, make it a solid covered run. Your chickens will thank you for keeping them out of the weather and if you get snow, it won't land in the run.