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How to increase size of thumbprints
Image > Resize
If you need to enlarge part of an image, make a selection of it and open it in a new layer. Make the layer a smart object and the stretch it with the layer selected.
I assume you mean thumbnails: The first image is a better view of the seocnd image.
Are you trying to increase the size of the thumbnails in the Organizer? You can do that by using the Zoom slider on the bottom-right of the window. Move it to the right to increase the size of the thumbnails.