Featured Image Set Specific Size

Hi, what size images should we be using for the featured image?

I was using all featured images of size 940px by 788px on my previous theme.

When I first switched themes to Colormag, I was using the widget TG:highlighted posts set to a size of 6 posts on the homepage. At first it was in alignment 2 rows of 3 images, then when I published a new post, same featured image same size of 940×788 but on screen it was larger than the other 5 featured images, so the images were no longer in alignment.

I removed that widget for now. But I do like how it looked when it was aligned nicely on screen and would like to be able to set it up again, I’m happy to change my images to another size or if you have another solution.

Thank you.