As a rule of thumb, you can collect around 600 gallons of water per 1000sf of roof, per 1″ of rainfall. Please check out this link for the average rainfall in your area.
Average temperatures and rainfall in US cities.
It is wonderful to go for the largest rain barrel size that would maximize the amount of rainwater collected, however any size you can afford and accommodate is good. Also remember to get a rainwater collection system that is closed, so that plant material and insects don’ t collect inside, and use the collected water within a reasonable amount of time (as needed during the days following the rain), so that it doesn’t get a chance to get stale.