Great question
Some languages are impossible to digest if they are more than a screenful - for example I could easily read your Perl that you recently posted, but if it's more than a function or two, it gets very hard. FORTRAN and COBOL were easy to read in 100 page listings. C is somewhere between - but in general it is much easier to read if broken into smallish pieces (say, a file per major function).
In general, the more a langauge relies on non-alphanumeric symbols, the smaller the digestible chunks - exception FORTH, which should never be more than 60 or so lines per chunk (because of the mental gymnastics needed to interpret the stack).
