The longer you leave a dahlia without dividing it the smaller the flowers will become. It doesn't have to try as hard to propagate itself if it has a nice big tuber clump to feed off of so it doesn't have to put out as many flowers or as big ones to attract insects & set seeds to ensure it's life.
The plant will eventually get weaker as well.
You will probably will be back to your bigger blooms if you divide it up. The pot shouldn't make a lot of difference if you watered & fertilized the same as if it was in the ground. I've seen AA sized dahlias grown to size in pots- spectacular!

Finding eyes is very difficult in the fall so just pack your dahlias away in a box lined with 8 folds of newspaper on all sides and cover the clumps with coarse vermiculite. Store in a cool basement, garage or crawl space but NOT somewhere they will freeze. 38-50 degrees is optimal. If you already have a storage method that works use it. When your tubers are in the house always keep your tubers covered with paper or other material so they won't dry out too much and start shriveling.

Check your tubers once a month to make sure they are not shriveling or rotting. Around the 1st of April bring them into a warm area and after a couple of weeks the eyes will start appearing and you can start dividing making sure you have one eye with at least one tuber.

Check out the GA Dahlia Society growing instructions at the link below.

Here is a link that might be useful: Dahlia Society of Georgia