If I just launch Excel by itself from the icon in the taskbar or Start menu, it properly opens at the same size I closed it. However, if I launch from an Excel file, either by double-clicking an Excel file or right clicking on the Excel icon in the Taskbar and selecting one from the history above it, the Excel window that opens is always full screen or maximized. I don't have this problem with Word or PowerPoint, both of which open files at the size I last closed the application.
I did not have this problem with Excel 2010. In fact, I haven't yet uninstalled it, because of problems like this with Office 365 and Excel 2010 still opens properly.
I use a 2560x1600 30" screen as my primary display (plus another display) so that I never have to run applications full screen, except for rare situations where I really need the application to take up that much real estate. How do I fix this?