How can my monitor size be used to track me? I mean, I guess if I have a certain brand, you can rule out some countries, but I'm pretty sure I'm not the only one with my monitor brand/size.
Well thats true, but the Window Size is not the only thing which can be tracked and will be tracked. The Windows size alone will not identify you.
But tracking nowadays is not only done by cookies and all the information is cross-referenced and matched. So with all Informations (from a normal browser) about your OS, Windowsill JS_Engine, Flashversion, Surfhabits, Key-press-pattern, Language and much much more can a evil person distinguish from others with a guarantee of 75% up to 99% ... and the Tor-Browser helps you to NOT give some additional Information.
Also, if most tor users leave the default size, that information becomes useless for identifying them.
In addition to the cross-referencing of window size with other bits of information like extensions changes, fonts, etc., consider a user who visits multiple sites at around the same time.
That user may be the only one with that particular browser window size visiting both site A and site B at that time. The size alone would provide a smoking gun for linking the visits, even across different Tor circuits, thereby damaging the visitor's cross-site anonymity.