Is there a way to change the size of featured images on posts? When I click on one of my blog posts, I can see it’s extremely large and inefficient.
The answer by Bruno is one way of doing it (by CSS) and that certainly works. The only thing to consider with this approach is that you're making the image fit the size, not making it smaller.
This is relevant when you consider the size of your web page in terms of the amount of Bytes being transferred. So for example if the source image is 1500px wide and 1MB in size, restricting the output size in CSS means the image is still 1MB to download.
So the other route is to select a different output size.
By default, Wordpress creates several copies of all Media Library images at different sizes, e.g. Thumbnail. So you could, within your code, select a smaller size source file, then use CSS to make precise changes.
You can also adjust the default size, and create your own custom sizes if you always need a version of specific dimensions - though note that you may need to regenerate your images if you change or add sizes as this isn't retrospectively applied.