Lightroom has 3 types of previews:

  1. The thumbnails. The size will depend on what you have set in the zoom factor. It applies to the Library module and to the strip at the bottom in the Develop module.
  2. The screen resolution images. The size will depend on the screen resolution and in the Library module, the preview size: Fit, Fill...
  3. The 1:1 previews. The 1:1 previews have the same dimension as the original photos.

Edit > Catalog Settings > File Handling

Set the following:

  1. Standard Preview Size: The maximum pixel dimension for the rendered preview. In today’s world, wide screen and high resolution, you should use: 2048 pixels.
  2. Preview Quality: The appearance of the previews. Either Low, Medium, and High. The higher the quality the more disk space used and the longer it will take to generate the previews. But in today’s world, meaning a fairly recent computer, you should use High.
  3. Automatically Discard 1:1 Previews: How often are the reviews deleted? That will depend on how you work with Lightroom.

    1. If you very rarely go back to “old” photos, expire them after a week or a month. Lightroom will regenerate them as needed.
    2. If you are low on disk space. expire them after a week or a month. Lightroom will regenerate them as needed.
    3. Speed is much more important to me than disk space. The terabyte hard drives are so cheap, that I can’t be bothered.

Lightroom: Catalog Settings > File Handling Tab

Lightroom: Catalog Settings > File Handling Tab