I'd consider a programming project "big" if:

• There's at least 2 programmers assigned to the project full time for at least a month, i.e., it's labor budget is over about 400 hours.
• Or it requires a users' manual that's takes up more than 1 page (12 point type, reasonable margins, A4 or letter sized paper)
• Or if the program requires extraordinary testing and verification (say, the control software for a nuclear reactor)