Hi hhrmp,
Sonar, as far as I know, won't let you change the font size for the printout. For all the cool stuff Sonar can do, its options for printing music are pretty limited, but generous since it's not a notation program.
You might want to take a look at more full-fledged notation programs... like
Finale, which even has a free version (or a $25 version that imports standard MIDI files that Sonar can export).
Or, if you really want to go nuts, take a look
Sibelius, which is the uber-notation suite, or so I hear from friends of mine. Great for composing your next avant-garde symphony.
If you just want to increase the font size on a lead sheet, check out the $25 Finale.