What size is this Dr Pepper shirt?

No standard size is stated. The seller gives a chest width of 19 inches and a length of 25.5 inches, so compare those to a shirt you own that fits well.

Is the shirt new or used?

It is used. The seller says it was worn once, possibly years ago, and has been stored in a closet since.

How do I check the 19-inch chest measurement?

Lay a t-shirt you like flat, measure straight across from armpit to armpit, and double it for the full chest. A 19-inch flat width is about a 38-inch chest circumference.

Is this an official Dr Pepper product?

The listing does not show a tag or licensing mark. Ask the seller for a photo of the neck label if authenticity matters to you.

Can I return it if it does not fit?

That depends on the seller's terms, which are not stated. Confirm the return policy before buying, since fit is the main risk with a measured, unlabeled shirt.