As far as I’ve been able to tell, it’s not possible to stretch an image using the Markdown syntax (though it does allow you to set a width or height to scale the image to proportionally) – but you can use a HTML img tag: 
(The image here is not only minimized, but stretched as well.)
The syntax for this is <img src="[…]" width=123 height=456> – for example, the image above has the source <img src="upload://ikwBl6z4zZKW3XIoX3jSPFWIwi6.png" width=64 height=48>.