new chook that's sneezing

Usual advice would be to keep new hens separate from existing flock for about 2 weeks, so that any illnesses like this one don't spread. But I think they are all in together already?

There seem to be 2 schools of thought about sneezes in recently moved hens, the stress of which can trigger illnesses in hens. One is to keep a careful eye and see if it clears up on its own. If they are eating, drinking and active I personally would give it a few days. Obviously if they deteriorate then off to vet.
The other is that it could be a respiratory infection that needs treating with antibiotics and so would need a vet trip, taking the effected chook with you and getting enough anti-bs for all chx.

Don't worry about bigger eggs, they often seem to get better at laying after the first few weeks and will lay bigger eggs.

If your hen is seriously ill, you could certainly try contacting the seller and explain the situation and ask for a refund.  Unfortunately because you have already mixed them in with the rest of your flock, if the chicken was seriously ill it's likely the disease will have spread so you would still need to treat the rest of your flock anyway.

I agree with henamoured that you should probably treat the sneezing one as if she has a common cold.  Make sure she has plenty of access to good food and fresh water (put extra containers out if the others are chasing her away), make sure they have warm, dry bedding, and just watch her to see if her symptoms get worse.

My Light Sussex was sneezing when I got her home, then she developed a rattly chest and I took her to the vet.  By that stage she was seriously ill, but fortunately antibiotics cleared up her chest infection and she made a full recovery.

Since then my other chickens have all had times when they have been a bit sneezy and/or had runny noses but they have all recovered by themselves without me having to do anything.