That's a nice decal!
I always make my torso decals on a template which already has the correct size, so I never need to resize them.
Here is my working method:
Perhaps this helps, the decals in here perfectly match the size of minifig torso's. There are also decals for dresses, shields, tiles and more. Save an decal image from the Brickshelf gallery, import it into MS Paint (which I always use) and erase the image so you only have the outlines left. An empty template ready to use!
Simply import the decal you've made in a word document and print it. Perfect size!