Margins in the edit window

I just upgraded to Scrivener 3.0 and am beginning to regret it. I opened my ongoing project and it fills only about 1/3 of the window. The margins are at least 3 inches wide on both sides of the text. Have spent over 30 minutes trying to get them to resize so my text fills the edit window as it did in the previous window. Cannot figure it out.
Please explain how to make Scrivener default to the maximum possible test width in the editor window.