Just how big is too big?

I recently read (somewhere on the internet) that the optimal size of a class is 70 lines of code (LOC).

My initial thought on such a size for classes was that it was extremely small, too small to be practical. Indeed, with some languages and frameworks, it is not possible to create a class with less than 70 lines of code.

Yet after working with "Immutable Object Programming" techniques, I have come to believe that classes of size 70 LOC are possible -- and practical. A recent project saw a number of classes (not all of them, but many) on the order of 70 LOC. Some were slightly larger (perhaps 100 LOC), some a bit larger (250 LOC), and a few very large (1000 LOC). A few classes were smaller.

The idea of smaller classes is not new. Edward Yourdon, in his 1975 work "Techniques of Program Structure and Design" states that some organizations set a limit on module size to 50 LOC. At the time, object-oriented programming was unknown to the profession (although the notions of classes had been around for decades), so a module is a reasonable substitute for a class.

What I find interesting is the similarity of optimal sizes. For classes, 70 LOC. For modules, 50 LOC. I think that this may tell us something about our abilities as programmers.

I will also observe that 70 lines is about the size of three screens of text -- if we consider a "screen" to be the olde standard size of 24 lines with 80 characters. That may tell us about our abilities, too.