Also in videos, if you click on the icon in the bottom right, the video pops out of the thread and follows you as you scroll down. Like this:
"He dropped the bat!!!!!!"
I just drag and drop and it defaults full size.
Also, if you select your image before pressing 'post reply,' you can adjust the size of your image.
Like so.
/I'm crawling my way out of an nWo wormhole.