My font won't stay.

I am using a Windows operating system and 2016 Excel.

I created a spreadsheet and clicked the top left corner to select the whole sheet, then changed the font to 15. I have save as and just saved, but each time I open the sheet only certain spots change to 11 point. 

I have been using the spreadsheet for over a year, so it has a lot of data and I have saved it a million times. I checked to see if those cells are set to "shrink to fit" and they are not. The whole sheet is set up as General. 

Any help would be appreciated!