My Sims 4 starting up in a tiny window

Nazj8  Try forcing the game to open in windowed mode.  (I realize it looks like it's there already, but it may not be.)  Go into Documents > Electronic Arts > The Sims 4, open Options.ini, search for fullscreen , and set it to 0 (zero).

If that doesn't help, please provide a dxdiag.  Hit Windows key-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop.  From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.