I assume that means that you Save and Close the document.
then return to the original document which shows text at size 11
So the document that you saved and closed earlier is not maintaining your edit / changes to the text size?
Without all the additional info regarding further pasting of additional text, it seems to me that Word is not saving your first basic changes when the document is Saved, Closed and then ReOpened.
That to me suggests that there may be an addon that is creating the problem.
Try opening Word in Safe Mode
Use the Windows Logo Key + the X key combination and choose run from the context menu
In the Run dialog box type
winword /safe (leave a space after winword)
Click OK or hit enter
That should prevent any third party plugins from loading. See if the behavior remains the same.
When pasting text into any word document you normally are present with three default paste options
Keep source formatting
Merge formatting
Keep text only
You might try experimenting between those three options to see which if any, provide the desired results.
Failing all of the above, my best guess is that your "Normal Document Template" (normal.dotm) is corrupt and you'll need to fix that.