reassign its width and height. in as3:
When you change the canvas size in Photoshop, you don't change the size of the existing contents. So if that's what you mean, you want to retain the current movieclip content at its current dimensions but make the overall movieclip as a whole larger, then you probably just want to add a background graphic (rectangle) to it sized to whatever dimensions you want. You can set the alpha property of that background to zero if you don't want it to show, or you can make it white for that white space you mentioned.
I don't know why you need to do this, unless the plan is to make the entire area mouse interactive for some reason.
I'm sort of in the same boat. I had another artists working for me. He built a walking animation for me, but the canvas size is way too large. It is causing collision issue in my SDK. Is there a way to do a batch resize of the canvas? So far I have had to resize each individual canvas in Photoshop, but this is time consuming.
I see what Ned's saying... If you add a new layer and draw out a rectangle to the "re-sized" dimensions and then move it down below your current layer, it will essentially re-size the canvas of that movie clip. And changing the fill to 0 would remove the background if you need to maintain transparency.
That's a good trick Ned. I fixed my issue a while ago though, but its nice to know for the future.