Assuming you used a 'Style' to create the text: -

If the 'smaller text' is all one 'Style' then 'Modify' the font size of that 'Style' and it will change all the text of that 'Style' at the click of the 'OK' button.

This does not work on text where you have manually overridden the 'Style' by manually setting the font and size of the text.