When you say width of the web page, do you mean the width of the artboard in Xd, or the width of the website content?
For the artboard size, you can go as big as you want, but I don't recommend going too wide. I keep my artboards 1920px wide or smaller, since clients most often open the prototype on their laptop, and the artboad will shrink to fit their resolution, which means it will look smaller on their screen. Also, if your monitor is smaller than 2560px, it might be annoying to navigate around the artboard if you view it at 100% zoom, as you should most of the time.
For the website content width, I don't go bigger than 1200px, rarely 1260px for desktops. The majority of windows laptop screens are still 1366px wide. If you are making a web app that's fully responsive with no margins on both sides, you can use the full res, but still focus on laptop screen sizes.
Hope that helps. Cheers!