As said above, you can use any image manipulation software to do this, from Microsoft Paint that comes with Windows to GIMP to Photoshop.
Every image editor allows you to resize an image. If you want it a specific size without any extra or less, you can simply uncheck the box on that screen that says "preserve aspect ratio" or the equivalent. This will, of course, warp the image which usually is undesirable, but you could also make it fit the minimum usable size and then crop it down (which might be referred to as cropping or resize canvas) to the exact dimensions you want.
This is very basic image editing that is better helped by looking up some tutorials for whatever software you're using.