It would be useful for designers to know what size browser windows users actually use (width x height a.k.a. length).

UMA currently records the size of the physical primary display (i.e., monitor for desktop platforms) into the system profile field.
https://cs.chromium.org/chromium/src/third_party/metrics_proto/system_profile.proto?q=primary_screen&147-149&l=148

However, there's no histogram recording actual dimensions used in practice.

This is particularly relevant for desktop platforms.  (On mobile platforms, Chrome typically takes up the whole screen.)