The default largest inline image display size is width 500, height 1000. That can, of course, be changed by using the source setting in the post composition window, but I'm sure hardly anyone knows about or uses that button.
The size seems odd to me. 500 px is pretty narrow for good display in most windows; even small mobile devices can handle larger or will automatically resize. And 1000 is pretty high, requires scrolling. I would think a width of 800 and height of 600 would work much better on pretty much all devices.
What is the reasoning behind the current settings?