One can track the window size and see what is used by visitors.
Even then... Is the window that wide because people like viewing things at that width, or because they've just arrived from a site that forced them to change to that width in order to see essential material along the right side? Let's assume for the sake of discussion that sites don't force you to make a window narrower than you like. (Hm. Interesting design challenge there.)
Tablets are in some ways amazingly retro. It's like jumping back 20-plus years to before MultiFinder (for those of y'all who even remember the term). One window at a time, one application, and the window takes up the entire screen.
Wrong. Reports from assorted humans on assorted computers had one line overflowing when I worked on this assumption. I've now set it to an outer limit of 600px unless the browser explicitly tells me otherwise.
* Tristram Shandy. If you've read it, you know what I mean. If you haven't, take my word for it.