When you open a new file in MS Paint (or right when you started it up), you get a large white box. If you hover your mouse over the very bottom right corner of that white box (make sure to scroll all the way to the bottom and the right before trying this), it should change into a resizing cursor. Now drag it to the top and left, until the image is really small. Then, if you paste the image in from somewhere else, it'll ask if you want to resize. Say yes.
Alternatively, instead of dragging the corner, you could press Ctrl-E, and set both height and width to 1, before pasting your image.