If you find that Lennart's suggestions don't help you might try Rolling Credits.
Rolling Credits lets you specify the size and font in your credits. In fact you can freely mix sizes, fonts and colors, add images that scroll with the text etc. The DV file created can be imported into iMovie and accurately represents your choices.
http://www.rqs.ca
Cheers,
David
Disclaimer: I may benefit from posts like this as I am the author the RollingCredits.
PowerBook G4 1.5 GHz 1GB / Power Mac G4 733 MHz Mac OS X (10.4.3)