XF 2.2 Scale article picture to fit


Article pictures don't scale to fit within the width of the preview. For example, I've got an 800 x371 article picture which XF has scaled to fit the height of the preview, but that means that the sides are chopped off, making it look all wrong.

I need XF to either be able to resize to the width, or have someone tell me what the exact width of that preview is, since the manual doesn't say. I'd rather not faff around by trial and error to try and figure out what it is.