WP won't let me Resize any images to Large format

Hi all,

My site is here.

My standard post layout is to have a image picture (620px wide) at the head of the post, usually resized down from a bigger image (e.g. 1000px wide).

Problem is that while the Editor allows me to resize images to Medium or Thumbnail, for whatever reason it doesn’t let me do that to Large (unless its a PNG file). The button for Large is filled up with blue and not clickable.

It’s nowhere near a catastrophic flaw (I can resize manually) but very inconvenient and any help resolving this issue would be appreciated.