Which size should I download?
Pick Full HD (1280 × 720) whenever it's available — it's the sharpest version and the one the creator uploaded. Older videos and some Shorts only have Standard or High sizes; the tool shows you which ones exist.
The smaller sizes are useful for previews, lists and places where file size matters.
Can I reuse a thumbnail?
Thumbnails are owned by the person who made the video. Downloading one to look at it, study it or use it for your own video is fine; publishing someone else's thumbnail as your own usually needs their permission.