Override image size in Casper theme

I’m on Ghost 6.34.0-rc.0+547f93e
Using theme Casper v1.6.1

In general, the 2000px max image width is fine, but there are times when I need to stretch beyond that a bit. In the article itself, the image can be whatever resolution happens by default; that’s totally fine. But when my readers click on the image, I need this particular image to be quite large. It is a long timeline of computing history events, and at 2000px wide the text is basically unreadable.

I’d like to be able to override the click-through image size on an individual basis, for one-off cases. Is there a way to do this?