Looking for CSS to adjust thumbnail size for gallery pages Momentum template

Site URL: http://www.michaelvanwegen.com

Hi, I am working on my photography website, which is using the momentum template. I use gallery pages which apply autocolumns. I would like to increase the size of the thumbnails shown on the gallery page (and have less thumbnails per row). Any ideas / suggestions how to manipulate the size of thumbs / number of thumbs per row for a gallery page setup? The image below shows the current (default) setup of the grid.

I have come across CSS to control / amend thumb size / number beyond the standard options for gallery blocks, but i really struggle to find a solution when it involves gallery pages. 

Many thanks. 

Michael