I was tempted to show off some toys I feel fatherly pride for, like my 500-line spreadsheet, but the only C program in this size range that I still use much is http://wry.me/~darius/software/req.tar.gz -- a rewrite-rule-based programmable calculator. Since it's >20 years old it's not at all what I'd write now.

(Toy spreadsheet at https://github.com/darius/vicissicalc)