When the picture is uploaded but the post is not submitted yet right-click on the picture and choose "Image settings" (or properties). Then specify the desired width/height in pixels (only width might be enough as it recognizes the aspect ratio). Not sure if the panoramas will do any better though - AR is very high...
It is a little confusing though... Do you just pick a random number & hope for the best? For instance I changed the width on this one from 500 to 750, but that was just a guess... 1000 was too big.