Hawkfish posted a photo:

Off West Bay Beach. The fish only allowed me to take this one shot before retreating into the coral, so the image is not as good as I would like. This was by far the largest lionfish I have seen ( maybe 18" or 0.5 m long), and it was much darker in coloration than any others. I'm not sure whether the dark color is related to its large size. See the previous photo in my photostream for a lionfish with a more typical color pattern.

Update a couple of days after I wrote this caption: I looked up on-line, and the largest Atlantic lionfish I could find was 47 cm from a spearfishing competition. So, I doubt the fish in this photo was quite as long as 18". More likely it was 15-16" in length.