How to add 1 Column with custom aspect ratio

Site URL: https://tangerine-bamboo-5w4z.squarespace.com/work

Hi there,

I am editing my site using template Beaumont. I would Like to keep the same style but change to one column but edit the aspect ratio to 16:3 so I can have 3 1080p photos next to each other of the Same project. Like this: rinayang.com 

I managed to find the code but that makes the photo very large and messy. 
 

thanks,

james