Custom Thumbnails for Portfolios

I love the portfolio feature of Jetpack and am using it to showcase my work. I have used a shortcode to place some of my featured work on the homepage, but would like to have a consistent thumbnail for each featured project. Is there some CSS code I can use to make the thumbnails all square, and a consistent size? Any help is greatly appreciated!

The page I need help with: [log in to see the link]