This one requires some consideration. Where you wish to display the Featured Images plays a role in your considerations.
When you add a Featured Image, by default, it shows at the top of the individual blog post. This image displays at the full width of the content (site width) or the full width of the Column if using Sidebars. So if you’re going with the default, it’s important that your Featured Images are the appropriate width for your site.
If desired, the Featured Image can also be turned off on Single Blog Pages on the Theme Options > Blog > Blog Single Post tab, and for the Archive pages on the Theme Options > Blog > General Blog tab. If you take this path, the Featured Image will still show as a thumbnail on the Archive Pages, and when using the Blog, or Recent Posts Element, and possibly much larger if using the Post Slider Element.
In this case, you can use a smaller featured image size. We recommend using an image at least 700px in width, as the Recent Posts Element requires Featured Images to be at least this size for them to be uniformly cropped when the images are different aspect ratios. In any case, even if you have uploaded a larger Featured Image, Avada is smart enough to only load the image size that is necessary for the layout, rather than just loading the full-size image. Please see our How Avada Controls Responsive Images document for more information on this.
If on the other hand, you want to have Featured Images on your single post pages, or you want to use the Post Slider Element, or add the Featured Images into your posts manually, you might want to upload an image size closer to the site width, or even bigger. The bottom line here is that Avada controls responsive images for you, and so it’s better to upload a larger Featured Image than one too small.