Actually, I just found Medium Glad Zipper Freezer bags in my cupboards. The dimensions are 17.8 x 20.3.
This will be what we are looking for? I only need it for a sample sized bottle of sunscreen. What happens if the bag is too big or too small?
Yup, it is the medium size. Besides that, we just noticed on our last trip to the airport (in Buffalo) that they have a table by the inspection that has empty bags on it, so if you are flying out of there and end up with the wrong size you can always grab one there.
I am in an airport several times a week; not every airport and not every terminal has these, so please do not rely on them.