How to copy game box art prints to wall posters?
You can either scan the boxes with a high DPI setting, or take photographs of them with a semi-professional camera. In both cases you'll end up with a high-definition digital reproduction.
You might have to perform some post-processing in software like Photoshop to properly isolate (crop and adjust the perspective of) the cover, adjust the white balance and/or the colours, and, if required, increase the size of the image and add some noise or light blur to disguise the (Ben Day) dots that are likely visible.
Send/take the files to a printer shop, make sure the colours are displayed correctly on their calibrated monitors (use the same colour profile - ask an employee) and you're about done.
We can't give legal advise here, but doing something like this for personal use is generally considered legal, as far as I know.