The first guess, naturally, is that Google is determining the actual pixel width of titles, regardless of how many characters it contains. If this is true, then I should be able to craft a title using a bunch of really thin characters–and then we can see how many of those characters get displayed in Google’s SERPs.
Based upon 469 pixels of width, I can cramp 112-characters in a page title. Theoretically that is. So this is the ultimate test.