Apparently no one here grows dahlias in containers much. I have chatted with a few people who do on other sites, and they use 3-5 gallon sized pots . You can probably grow them in smaller pots, but once they get bigger you would have to water them twice a day.
Many of the places that primarily sell dahlias have groups they list that are especially good for cutting. Generally the smaller flowers (BB sized) produce more blooms and will last longer in vases. With ball dahlias you won't have the problem of down-facing blooms. What the people who show dahlias are looking for is slightly angled, mid way between up-facing and side-facing. So thats what the breeders aim for.