Interestingly when sizing a terminal window these WM's show the size in columns and rows and when sizing a "real" window it shows the size in pixels.
Probably an option somewhere to turn it on in newer environments but it's been a while since I used anything but cwm[1] (which does show the window size on resize) so I can't be sure.
[1] calm window manager - https://en.wikipedia.org/wiki/Cwm_%28window_manager%29
IIRC there's a window property the app can set on itself to tell the WM of its "resize increment", as well as a "base size". Since the main use case for that is terminal windows (anything that displays fixed-width text), I think most WMs assume that anything with that set on it cares about rows and columns rather than pixels.
I use a Slackware box at work and was delighted when I found a Linux port[1], now I use it everywhere except on Windows.
I don't see a tooltip that shows the window size. I am using Xfce.