I’ve been designing websites for several years now. Long enough to remember 800 x 600 pixels was the dominant browser display size when I started.
I was curious what the current standard is and started poking around online to find some info. After a bit of a lengthy search (many of the links date back as far as 2001), I finally found it.
Answer: There really isn’t one. It’s kinda spread out all over the place. W3 schools has some current as well as past statistics on this very subject. It might help you in your design decision-making. W3 Schools – Browser Display Statistics