He wants it for a design template to hang in his room.
So, his intent is to make a print of it to hang on the wall. Sounds like you should be selling him a nice print in whatever size he wants to hang on the wall, based on your regular print prices.
It makes me wonder why he even wants the digital file.
However, if you really want to sell the digital file so that he can possibly edit it and print it himself, I'd again determine the intended size from him, write up a contract that specifically details the ONLY copyright use you're allowing based on what he says he wants to do with it. Include your price for a print of that size (minus your regular printing and shipping costs, if you feel generous - BUT - The fact that he'll have the digital file and can then make copy after copy as they fade or get destroyed without repeat buys from you should be taken into consideration). When the contract is agreed to and payment is made, work up a JPG file appropriate for the size he wants to print, and send it to him.
It reminds me of the kind of transactions that one engages in when buying stock photos online to work with, which are typically delivered online via JPG or TIFF file. If you buy an image to use as a background or a prop from
Fotolia.com for example, the price varies depending on the original photographer/artist's value of his/her work and the size/quality needed. No matter what though, the buyer is restricted to use the image only as the contract agreement outlines.
Keep in mind also that stock agencies sell the same image to a lot of people/companies over time (the photographer/artist hopes), and they have a large enough audience and advertising resources to help make that happen. As such, it's not unusual for them to sell digital images for literally PENNIES each to users who buy in bulk. You, on the other hand, may only sell this image as a digital file just this once, and will have to price it accordingly.