“Large” Image Size?

I need an intermediary between “medium” size, which is rather small (but larger than a thumbnail) and “full” size, which displays the images as they are just shrunken in HTML. I need to use big files that can be accessed as big images if clicked on, but it’s slowing down my main page because everyone has to load all the huge images. Is there a way to get WordPress to make “large” images like it does “medium” images such that the size of the images will line up with the maximum size for images in the CSS? I want the images to show up as they do now with the HTML resizing, but to actually be smaller images rather than just shrinking the huge images with HTML.