Blog images should be no smaller than 300 pixels wide by 600 pixels high. Larger sizes may be appropriate for some blogs, but they should be used with caution because they can take up a lot of space on a page and may not be well-suited for all content. When choosing a size, consider the type of blog and the audience it is intended for. For example, lifestyle blogs catering to women may want larger images to fit in more on a page.
On the other hand, technology blogs that focus on product reviews may prefer smaller images that are easier to read. Always make sure to test different sizes in a browser before making a final decision.