Edit ==> Preferences ==> Appearance will let you change the font size, both in code view and in book view.
This assumes you don't want to actually change the font size for the book itself, for which you would use css.
The Preferences just let you set a default "to display within Sigil".
Also, if there is a specified font size in your css, this will override the Book View setting; Sigil will instead use whatever font size your ebook has.
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