Sneezing Chick

chickens don't catch colds like we do so she could have one of the following:-
something stuck up her nostrils - check and remove if this is the case,

hayfever - you can give a little bit of piriton but the amount needed I wouldn't like to guess,

dust allergy - are you using dust extracted shavings? Sawdust really is not recommended,

respiritory infection - is her breathing rattly, has she got snotty nostrils or bubbly eyes? Has something stressful happened recently such as a moult, just coming into lay or new birds introduced? If this is the case then a visit to a vets is needed as soon as possible for antibiotics. You will need enough to treat all of them.

I would be wary about using Piriton unless you know the correct dose as overdosage has the potential to cause excitability and convulsions.
Loratidine has a much larger margin of safety but I would prefer to discuss use/ dose with a vet.