I usually like the retro style using sprites at their native tiny sizes and change the fullscreen in browser project property to letterbox integer scale which seems like it may give you the desired results looking at the screenshot.
Otherwise you should scale up each sprite the same amount, it starts getting fuzzy when the size of the image being displayed has to be stretched from the original image. Depending on the window size you are targeting I used a standard x10 to keep it simple but you may need more.
Hope that answers your question.. I fought with the same issue for a long time before figuring it out.