If you paste text into the editor from another source and it changes the font size, Ctrl+Z to undo will not undo the change in font size.
Open a new scrivener project.
Type one letter to see what size is normal.
Delete that letter
Open the pdf manual. Copy some text that is a heading of some sort (bold and larger than normal)
Paste that into scrivener
Ctrl + Z to undo.
Start typing… your text will be the size that the pasted text was.