I'd like to introduce you to this amazing little tool made by Jaex called Hearthstone Resizer. As you can probably tell by reading the name, it's a tool to resize your Hearthstone window to fullscreen.
Since Hearthstone does not support a borderless window-mode this tool might come on handy as it allows you to still have your window that way.
It should not be very hard to understand, but I still made a commentated YouTube video for you guys:
Please note that this is not my program, as I only modified the style and added another function. All credits go to Jaex, !
Download: