Your tutorial seems to be a little bit old. You should have a look at this manual : http://prawn.majesticseacreature.com/manual.pdf and/or check the railscasts episode : http://railscasts.com/episodes/153-pdfs-with-prawn-revised.
You can use the font_size
property to define the size of your text like this :
font_size(25) { text "Even bigger!" }
You can also do it like this :
text "Single line on 20 using the :size option.", :size => 20
Or you can use a block to apply different font properties to your text :
font("Courier", :size => 10) do
text "Yeah, using Courier 10 courtesy of the font method."
end