Hello,
You need to try several combinations of image sizes to get the best one. There is no recommendation. You need to take one wide image in the big column and one high image in the narrow. You know if you set the equal width for both columns the images would get the same. If you, for example, have three columns of the equal size and use equal sizes images it looks nice. At the same time when you try to create something like a masonry, you will have to process image sizes until you find the best combination.
Best Regards