Hi there,
You can set the image size parameter in the gallery shortcode, like so:
[gallery link="file" ids="11493,11496,11490,11485,11455,11500" size="large"]
The size determines the image size that is loaded, so large is likely going to be ideal for you in a 3 column set up. You also want to set the link to be “file” so that it opens in a lightbox.
In terms of the gallery slider page, we have some updates planned for this, but don’t have this page linked into the theme as of yet. What would you like to be able to set for it?
Hope that helps!
– Ed