The pots Romence Gardens uses are industry standard trade sizes. The description is the trade name and recognized as such to be the size it is.
Here are the specific volumes of the pot by size.
4 1/2" pot: Volume = 1.31 Pints / 619.84ml. Diameter is 4 1/2 inches at the top of the pot.
Quart Annual Quart: Volume = 1.94pints/ 917 ml. Diameter is 5 inches at the top of the pot. Perennial Quart: 4 x 4 x 5 inch square pot: Volume = 35.1 Fl oz. / 1.04 Liters
Gallon Pots: Annual and Premium Perennial Gallon: Volume = 2.72 qts/2.57 liters Regular Perennial Gallon: Volume = 2.513 liters