3 - 5 lines of code on a presentation slide (and really, really big) is probably all that you can count on being legible to everyone in the audience.
Since you're not given even twenty lines for each function but five lines for the whole program, the best way to get it all fit the slide is, of course, to golf!
On a more serious note, I agree with this: put the key points on the slides, and very, very little cruft. No-one wants to read long slides. What this means with code depends on the case, but I would say removing error handling, reading input and output, and in general anything extra that is not strictly relevant (unless those listed are what you are talking about). The less you have stuff per slide, the better.
One tip: if you use page numbers in slides, don't use the form "43/300". Instead, just put the current page number or leave it out altogether.