A couple thoughts about the letterboxing and what I experienced:
While the TBB is doing a good job at obscuring the fingerprint vector of window size, there were a couple observations I wanted to share with our community.
Some desktops, Gnome3 in particular, but there are others, have certain display features like ‘docks,’ and ‘title bars’ to name two that can really pigeonhole the user into certain groups. I know we use XFCE here in Whonix, but even so, there is a lot of flexibility in how we size panels, where we place them, how big they are compared to other known panel applets for other popular destops and things like that. Also, we cannot forget the host DE and how its features may affect the determination for the maximized window size.
If the user does not use their virtualizer in full-screen view but maximizes the TBB window anyway, he or she ends up with some unique window dimensions. This can be true of any desktop. For example in virtualbox, default settings have the top of screen machine tool bar and the mini-bar on the bottom underneath the panel. If identities are not switched (or even if they are) the irregular window size can be something that can maybe not outright identify you but narrow down the possibilities
It won’t be absolute, but over time it can be problematic.
Example: it may be that using Gnome3 on a 1366x768 laptop produces a unique maximized value that would be different on say LXDE or XFCE on the same 1366x768 screen.
At least when the TBB window was default set at 1000x800, 700, etc. then the pool of users was much much bigger and the various idiosyncrasies of each unique desktop environment was much harder to pin down to a certain person or persons.
Maybe for now to just be extra cautious it would be better to not use full-screen view just until some of the unknowns are better understood or worked out.