That's not something we do with {{retitle}} (and we would use that template, not DISPLAYTITLE directly) at Tardis, as of 2018 anyway. And I don't see a need to start.
For one thing, I don't think it's unseemly for a long title to take up more than one line. That's kind of the beauty of a long title.
For another, where would we draw the line? How could we determine which names need smaller-text titles, and which don't? The name may take more or less space on a line, depending on browser and window/screen size.
The way I see it, page headers accommodate for long titles just fine, on their own. Long names don't run on ad infinitum, past the buttons and then the margin. It automatically allows for a bit of a soft line wrap.
And if I added {{retitle|{{sm|{{PAGENAME}}}}}} to the aforementioned Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union, it would still take up three lines for me.
And I'm seeing Eleventh Doctor (The Curse of Fatal Death) already on one line, on my computer, so, again, where would we draw the line?