I've seen 10k+ SLOC functions written in C, and 20k SLOC functions written in Fortran, so it wouldn't surprise me if people created ones as big as bluGill describes.

The C was almost always written by EEs who learned that function calls were expensive and so they minimized their use of them (this was their stated rationale, not me guessing). What amused me was that every time I tackled one of those things I'd reduce the line count by 70-90%, and usually at least double performance, by using a bunch of small functions to encapsulate the repeated logic. Compilers inline well, and have for quite some time.