I think it'd depend on the size of the Dahlia. We grow miniature Dahlias in large (12" or bigger) pots on the patio. But I think it'd be difficult to grow the bigger varieties in a pot unless it was a dwarf plant. I can't imagine a dinner plate dahlia in a 12" pot.

I grow all my dahlias in pots. They are started indoors in a heated garage until they can safely go out June 1. I will transplant a few into my beds, but will sometimes just sink the whole pot in the ground. MOST of them stay in their pots all summer....Easier for me to get good size until they can go outdoors and easier to unpot and store at the end of our short summer.

The bigger the dahlia, the bigger the pot. As a novice, I used regular nursery pots (including for dinner plates), not realizing the issue with staking. I now use sturdier plastic pots for most of my dahlias. They are 9" wide by 11" tall. But I use #7 nursery pots for my dinner plate and tall dahlias. This is what the greenhouses sell their dahlias in up here. They are 12" wide, but 13" deep....An issue we have up here because of our 24 hours of daylight in summer is that everything grows taller than projected, including dahlias.

That gives you two schools of thought geographically...LOL