Need help changing size of screenshots
 

I'm able to posted screenshots in threads (upload the jpegs to mediafire, then use the link to images function in the new thread toolbar). However, my screenshots all come out relatively small. How do you change the size of the screenshot to make it appear larger in these threads?

Note: when I select the link in Mediafire, I have tried both "small" and "large" for the image size, with the same result in the subsim thread when I post.