If you have any pictures in a folder, you can display them as thumbnails by clicking on the View menu and then click Thumbnails. The miniature size of default is 96x96 pixels. It is possible to enlarge by modifying the registry.
Here the tricks

  1. Click Start, Run, type regedit and click OK.

  2. In Registry Editor, navigate to the

    HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Explorer.
  3. Click the Edit menu, click New then DWORD Value.

  4. Name the new value ThumbnailSize.

  5. Double click, select the decimal and enter the pixel size of thumbnails. Enter eg 192 for miniatures twice as large.

  6. OK then close the Registry Editor to apply the change.