What you need to do is print at a particular DPI (dots per inch) value. MSPaint doesn't do this, but better image editors do. A good one is IfranView
So to set DPI in IfranView:
1. Open your picture.
2. Click the "Image" tab at the top.
3. Select "Information".
4. You'll see two boxes with "resolution" next to them. This is the DPI. Depending on your template, it'll probably be either 500 or 300.
5. Type in the DPI and print!
Post your decal at its orginal size and I'll tell you the DPI value.
Also a useful thing to note is that minifigs aren't measured in inches. They're designed in Europe, so they're metric.