Oh "bloody potato"(? lol), sorry, I thought I was posting in General. That was bound to happen sooner or later though when having to many tabs open lol. Thanks for moving it.
I have to say, at least for me, it would definitely be more convenient, also it wouldn't have to mean an end to the "Canvas Size" dialog box and the work already done on it.
Also I don't think it would get in the way if implemented in such a way that people don't accidentally resize the canvas when they want to resize the picture/layer or vice versa. For example a button on the "Canvas Size" dialog, lets say "Use mouse to resize", which when clicked would make the grab points appear around the canvas as they do for layers and the user can resize it from there, pushing Enter accepting the changes and Esc cancelling them.