How do you measure a small, big, very big project?

I am reading Code Complete as well as a couple of other programming books at the moment. The size of the project affects a lot of other parameters that you need to take heed of in planning, coding, automating tests, etc. I was wondering what people on SO generally use in the real world to categorize a project in its correct "weight class"?

Is it lines of code? Number of external interfaces? Necessary pages of documentation?

I would say it's the amount of development effort required. Taking a team of six developers:

• Small project - up to 6 months
• Big project - 6-18 months
• Very big project - 18+ months

Everyone will have a different opinion though.

Edit

I was thinking about how those values would change for a 1 developer "team". I think they would be along the lines of:

• Small project - up to 1 month
• Big project - 1-3 months
• Very big project - 3+ months

This seems to suggest that for small numbers of developers the rule of thumb for project size might be:

• Small project - up to 1 month per developer
• Big project - 1-3 months per developer
• Very big project - 3+ months per developer

I doubt this scales beyond 6 or so developers though as the number of communication channels starts to drag on the potential development time of each person. Effectively leading to less work done per month per developer the more people you have in your team.