Hello @kkgoss@yahoo.com

If the blog posts layout is set to as masonry then, the masonry layout automatically fits the images to the window as per the size of the image you upload. There is no such fixed size of the image.
However if you want all the images of same size then you need to upload images of the same size for all the posts as you wish.
so, if you upload image with lower resolution then also it automatically fits into the windows as small image and if you upload image with higher resolution then also it automatically fits into the window as large image.
So if you wish the images of blog posts of same size then same size image should be upload.
Hope this will help you.

Best Regards!!!