Thumbnail and Preview quality depends on a couple factors -
1. The Width of the Display Screen, and,
2. The Width of the Original Image.
For thumbnails within the library, assuming the width of the image is greater, then the quality will be as shown here-
|Display Window Width||Thumbnail Size|
|<= 300 Pixels||240 Pixels|
|300 - 400 Pixels||320 Pixels|
|400 - 625 Pixels||500 Pixels|
|>= 625 Pixels||800 Pixels|
For previews within the library, the image will be at most 2000 Pixels.