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If you really know every single line of the program, so that you can explain why it is there and what it does, and you can tell the effect of changing it, then the program is not big.
I have never been working with a program in excess of 30 KLOC without, every now and then, scratching my head, asking "Why the *** is it programmed like that?" - even when it is programmed by myself. So it is a big program.
I guess that for me, the big programs start somewhere between 10 KLOC and 30 KLOC, depending on the complexity of the code.
With one exception: Any APL program exceeding 30 lines is big.