Holding the SHIFT key while in free transformation will let you make the
image bigger or smaller and keep the shape.
If you have a photograph or scanned in a negative you can get rid of most of
the dust and scratches by going to the remove dust and scratches option in
the filter menu.
If you are merging two images into one with the layering system you can
change the opacity of the top layer to fit them better and erase around it.
The opacity controls are located on the layer pop up window on the right.