Help with photos

For some reason, when inserting photos, I'm not able to choose "Large" when choosing the size to display the photo. The way I have my layout set up, I can't insert a photo larger than 550 pixels wide. I uploaded a photo that was about 800 pixels wide, then went to insert it. My only choice was a medium at 300 or the actual size. The large option was greyed out. Why would this be? I have my wordpress media options set to max width 550 for large photos; so why wouldn't this option be available to use??