yeah the setting of _xscale and _yscale is percentage based
so 800 sets it to 800% of original.
You had the right idea with _height and _width but thats
pretty old flash 4 style code so i'm guessing you're learning from
an old book.
If you've got a copy of flash later than 6 or you're using
the latest free demo look up "MovieClipLoader.onLoadComplete" in
the help section and use that code to load in your movie/image.
Then in the onLoadInit handler put in the code to set the
size as in the example.
target_mc._height = 800;
target_mc._width = 600;
If you have any problems converting your loading script to
the example just post it here.