Image Size Gallery

This is the documentation in the WordPress Codex for what you want to do.

You want to scroll down the page to the options where you will see the “size” option. You can change the size by adding the size option in your gallery shortcode.

Ok thanks, Now the question is where would I place this size=medium tag?

size
specify the image size to use for the thumbnail display. Valid values include “thumbnail”, “medium”, “large”, “full” and any other additional image size that was registered with add_image_size(). The default value is “thumbnail”. The size of the images for “thumbnail”, “medium” and “large” can be configured in WordPress admin panel under Settings > Media. For example, to display a gallery of medium sized images:
[gallery size="medium"]

If you click on the Text tab on the upper right hand side of your post editor window, you’ll see the gallery shortcode, which looks something like this:

[gallery ids="8,9,10,11,12,13"]

Change it to this:

[gallery ids="8,9,10,11,12,13" size="medium"]

Let us know if you have any other questions.

Thanks,
I changed my gallery shortcode in https://www.travel-ascending.com/las_vegas to read [gallery columns="4" link="file" ids="453,454,456,460,461,462,463,464,465,486,487,488,489,490,491,492,493,494,496,497" orderby="rand" size="large"] , yet I see no change to the relative size of the images displayed. Could this be due to the original size of the image?

A second question regarding this topic: Is there any way to make the default size large for all galleries without changing the shortcode individually on each page?

It could be due to the original size of the image. You can check the dimensions by looking at the pictures from the Media menu.

Also, it looks like you’re using the Gallery plugin. Have you tried using Jetpack or another gallery plugin?