Magnolia - Recommended Image Sizes
Featured Image Size
For your featured images, images must be no smaller than 800x1050px. Larger is fine. That size will ensure your images display correctly in all areas of the theme (front page, blog page, category pages, etc).
Post Image Size
The width of the blog post column is 775px. For the clearest possible display, we recommend uploading images roughly twice that size (1500px is great).
If page speed is a concern, you can upload images right at 775px, but your images will be fuzzy on retina devices. Larger images don't really affect page speed all that much, unless you have an excessive number of images in one post.
If you're using the full width post layout, the post column is 870px and images should be at least that size.