Making project images smaller in a one-column gallery

Hi! My website is www.othi.la // password is Print01

On my home page, on the portfolio section, I managed to make the project gallery have only one project per column (as supposed to the template's default minimum of two) using this code: 

.portfolio-grid-basic {
  grid-template-columns: 1fr;
}

That is working great. However, now I want to make each project image smaller, so that there is more space between the images and the site's margins.

What would I have to add to the CSS to make that happen? 

Thanks in advance!

Hi,

You mean this section?

If anyone has this problem, you can use this code to Website Tools > Custom CSS

div#gridThumbs {
    max-width: 700px;
}