I've been thinking about this for a while, and now seems as good a time as any to bring it up. I agree that 250px is a good conventional size for most thumbnails, particularly of portraits of individual characters and individual objects. But this size becomes uncomfortably tiny (especially on my 1920x1080 monitor) for scenery views, group shots, etc., where individual subjects and features are necessarily smaller parts of the picture. That's why, for these kinds of images, I've unilaterally set most of these thumbnail sizes to 400px instead. In Turtlepedia's standard layout, it still leaves ample room for lede text to flow adjacent to the thumbnail. Note that these apply to illustrative thumbnails accompanying text and infoboxes, not to gallery sections themselves. Additionally, particularly expansive panoramic views that are much wider than they are tall (and these are not super-common), when used as illustrative thumbnails, could conceivably be set upwards of 600px wide and maybe even occupy their own lines, only briefly interrupting text.
Now, this works when there isn't a disagreement or edit conflict, but considering I've been doing the 400px thing without official sanction, I think it only reasonable that I raise the issue and, perhaps, propose it be elevated to a specifically permissible guideline, reliant on how much the image would suffer if shrunk to 250px or smaller.