In PNG 8 bit format, I think an ideal size is between 1024 and 2048 pixels, which should fit under 200kb. It will be scaled down to whatever thumb format is used on the page, but still readable.

My hunch is most people will expect to see the map straightaway. It is a significant spoiler.. but then the dungeons are not on the front page. I think it makes sense that players specifically following through to a dungeon page expect to see spoilers. It is not possible for us to write a dungeon page littered with spoiler tags anyway, and it would be more annoying than helpful for players who want the information in the first place. Dungeon articles is one of those pages.