Depends on the size of the dahlia but last year we grew a selection of very tall dahlias and each one had a 30cm or 40cm or 50cm tall pot and they did brilliantly.

As a rule of thumb, plants will need a pot 1/3 as wide and deep as they grow tall.

As big as you can manage! And heavy ones too, if possible. We had some that kept blowing over during late Aug/Sept once they were fully grown.

thanks all for your comments. Volunteer, it did occur to me about the pots potentially blowing over. i'll make sure i get hefty ones. Inverglen the dahlias range in height from 90cm to 110cm. where yours this kind of height? Good to know they did well for you in pots.
Tugbrethil - thanks for the tip!

The sort we grew were not particular names but were the dark leaved sort and grew up to a metre high by Sept! Mix some food granules into the compost to keep them going all summer.

thanks volunteer!