You'll need to assign a listener to the MovieClipLoader and wait for the swf to load. Once it has loaded you can set its dimensions as you like. Look in the help documents for the MovieClipLoader.addListener method... there is an example there that should help.
Another option would be to make the holder the same size as the file you'll be loading and scale it down to the size you want it to appear as. When the file is loaded into it, it will be scaled per the holder.