I've just noticed that the window size popup is showing in the tabs I've newly opened after the reinstall, meaning that the reinstall has fixed the issue.

Thank you for the reply. 

I went into the Settings menu and confirmed that the option you mentioned had already been unchecked.. So I reinstalled the extension but ended up with no indication of the window size still.

I'm quite sure that the grey pop-up indicating a window size used to display on the right bottom corner of my Chrome browser window when resizing the browser window, but now it will not show even though I'm resizing the browser window manually.

I've viewed the screenshot too but don't have a clue about what else I can try.
If there is other possible troubleshooting, please let me know.