I haven't tried this in 4.4 just yet but in a 4.2 site we are just using ?size= and then the size to constrain to. With this approach, it will re-size the image so that it won't be any wider or taller than specified size. There really isn't anything that I am aware of that allows for specific height and width properties to be specified.
I like this approach as it keeps the proportion and just determines if the image is a portrait or landscape and then uses the higher value to constrain. The re-size would happen on the server side so you actually got an image with the smaller size.
So for your example, you could use the following:
This would constrain the image to a max of 640 either in height or width.