MS Paint will tell you an image's size, as will any other graphics editing app. Or you can right-click an image online and choose to see the image's info or properties.
The sizes are all the same in any image editing program. You literally don't need to know anything else besides 300x100. Just go into Photoshop or Paint and create a 300(w)x100(h) image and you're done. Add stuff to it.