The Sims 4 only opens in a tiny, minimized window that I can't make larger

• Do you have Origin Beta versions opted in ?
• Could you do this: http://help.ea.com/en/article/how-to-use-origin-error-reporting/ then post yor Origin Report ID in this thread
• As a workaround you can try if you can turn of Origin in game: http://sims4.crinrict.com/en/2014/09/deactivate-origin-in-game/

Please get the above info first before trying the work around.

Do you still have the issue ?

I had this same issue. I had to close it in task manager three times. Then I opened it through origin being careful not to touch anything in the first minute while it opened into a larger window.