Adjusting Image Size in A SubWindow

I have created a subwindow which I want to put an image plot in. But since my horizontal axis range is quite small the plot looks basically like a vertical line. It does have visible width, but I cant figure out how to resize it without changing the values on the horizontal axis. Is there any way to control the size of the Image? Additionally, I will be making plots of various sizes, is there a function that I can use to set all images that are generated in the subwindow to the same size? 

Thanks!