When you're having an image printed, just how big does it need to be?
We already have an article that covers, extensively, just how big your images need to be for web use. It goes into the difference between dots per inch and pixels per inch, it looks at image width and height, it examines the impact of quality on your screen renders, and it skims over file size. If you regularly use images on the web, we suggest that you go read it.
But what the article doesn't cover are image file sizes when it comes to print. Thankfully, the stock site Alamy has just the infographic for that.