I'm sorry to hear about your chick
Do you have photos of your chicks or a video of them sneezing?
How often are the remaining chicks sneezing? Does the sneezing coincide with any activity like eating/drinking, dust bathing, cleaning the bedding, preening, etc.?
Are they inside a brooder or do you have them outside in a coop - how much ventilation do they have?
Sneezing in itself, unless it's continuous, can be due to dry dusty bedding/feed, poor ventilation, molds or just sometimes they need to sneeze to clear their nostrils. Most of the time, if you are dealing with a respiratory illness, there will be other symptoms to accompany the sneezing (mucous, watery eyes, coughing, facial swelling, lethargy, not eating/drinking, etc.).
Some viral respiratory illnesses like Infectious Bronchitis can be fairly mild and the only symptoms you see would be sneezing, a reduction in activity and possibly not eating as much - unless complicated by infection - IB will usually run it's course in about 1month.
Antibiotics are helpful when treating secondary infections due to respiratory illness, but will not prevent a bird from getting sick and should not be used prophylactically . Treat only birds that are showing signs/symptoms of respiratory illness. You may want to try the VetRx first, check to make sure there is plenty of ventilation and reduce any dust/respiratory irritants before considering treating with antibiotics.
To answer your question - injectable Tylan50 dosage is .20ml per pound of weight. This can be given orally 2x a day for 5 days. You will want weigh each bird that you are treating to ensure you are giving the proper dosage.
Keep us posted.