The dosage for Sulfadimethoxine is 1/2 to 1 teaspoon per gallon of water depending on how severe the case is. If your birds are really suffering with gummy mucus coming out of the nostrils, gummy eyes, etc...I would go with 1 teaspoon per gallon. Make new batch daily for no more than 7 days. Sulfa drugs are too hard to use past this length of time.
If your birds are still suffering but improving after the week mark, you can try switching over to Duramycin. Not AS effective on Coryza however it should finish off this bacteria and can be used for up to 14 days.