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?