There are two textboxes. One from the drawing toolbar and one from the
controltoolbox toolbar.
I put a textbox from the drawing toolbar on a worksheet. I entered some text
and then right clicked on the textbox border and chose Format text box. One of
the tabs was Font where I could the work.
(I could also highlight the text within the textbox and rightclick and choose
format text box. Only the Font tab appeared--where I could do the work.)
When I put a textbox from the control toolbox toolbar, I could show that
toolbar, click on the design mode, rightclick on the textbox and choose
Properties. There's a Font property that you can change from the properties
dialog.