How to resize a window on launch?

There has got to be an easy answer to this. When I have several windows open at once I sometimes one at a different size. Then later on when I launch a new window it opens at that odd size and I have to manually resize it. Typically I like my window to be about 85% of my desktop. How do I get windows to STOP launching the window at the odd size?

What's the odd size?

I might have a window just barely large enough to show a YouTube video (and ONLY the video part) while I am working on a larger window like typing these responses. Then I can go and close all the windows, and later when I start opening windows again (sometimes - not always) the newly opened windows are the size that I had been playing the video in.
Any suggestions?