Previously you would get different results, if I recall correctly, depending on where the image is when you grab a snapshot.
Library, main timeline, secondary timelines, Each may yield a different size image. I'm not sure that is true in the more recent versions of PD. I think you will get full resolution everywhere in PD16.
Be sure in Preferences>File tab, you have full resolution checked, if the choice is there.
A snapshot is a photo, so I'm not sure what the difference is for you.
Check the properties of the snapshot. I think the Library is the best safest bet.
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