I am trying to print decals, but I don't know how the size of the torso or any other parts are supposed to scale in my Microsoft Paint.
This link for decal specs.
But honestly, the issue is mostly about your program of choice. You can't resize in anything but pixels, and as the other responder said, your dpi really means the entire problem, because each dot is a pixel, and it can print waaay off. There is a free, proven-safe program (I have it on all my school computers too,) called Paint.net, which allows you to choose your size in centimeters/inches for your graphic. Now all you have to do is measure your torso, and put the art at normal size onto your paper.
In Microsoft Paint? Sorry, man. It just won't cut it. To use a measure, you need to have a program that understands a measurement.