Using this method doesn’t use code, and you can easily revert back to the default settings at any time. It does take time though, and time again, if you decide to revert back. You may want to look through your theme to see how images are used in your theme, and decide whether the larger sizes will look well in areas other than the galleries.

media-settings

  1. Click on Settings and then on Media
  2. You can edit the Thumbnail size to be something larger, say 300px X 300px. You can leave the Medium size at the same size or change it to be something larger, like 600px X 600px, depending on your theme. Save.
  3. If you already have images in your Media Library, you will need to resize them.
    1. Install the plugin Regenerate Thumbnails.
    2. Go to Tools and then Regen. Thumbnails.
    3. Click the Regenerate All Thumbnails button. (Note that this can take a long time, if you have many images, and you must leave that window or tab open in your browser. You can still use your browser in another tab or window.)
  4. Now go to your post and insert (or edit) your Gallery. Choose 2 columns, and click Update Gallery or Create Gallery. See how to create a WordPress gallery.
  5. Note that if you want to return to the 150px X 150px thumbnail size, you can just edit your Settings > Media again. You will need to use the Regenerate Thumbnail plugin on the images you newly uploaded. You may also want to go back and edit your galleries to change back to 3 columns.