Hi there,
Thanks for posting in.
Doing it with exact size in your mockup design isn’t possible since the theme is made to be responsive. What you can do is just modify the columns to match the ratio of your mockup design,
Example, change your row columns to 2/3+1/3, this will make that row to display 2 columns but make the first column wider (2/3 of the total width of the row). Or 3/5 + 2/5, it’s the same, but yes, you can’t make it exactly as displayed in your screenshot. Unless you’ll fully customize or create your custom columns.
Then within your row, toggle on the margin-less option, and set backgrounds to your columns. Just inspect your rows and columns and the options are available there 
Thanks!