With the image opened up in PSP, click on "Image" then "Resize".

Make sure there is a dot beside "Pixels" (not percentage). Next, input your size in the "width" box. (this is where you will put 400, 450, 500 etc.) Don't worry about the "height" (read the next paragraph)!

At the bottom of the box make sure you have Resize Type as "Smart Size" and that the two boxes underneath are checked. (Resize all Layers" and "Maintain Aspect Ratio".) That way if your LO is not quite equal on length and width, it will adjust the size perfectly.

Click "OK" then save your new file!