It really depends, some companies are just trying to get an idea of what your code looks like, other's are looking at it not just for form, but a demonstration of your problem-solving skills.

With the former, just about anything will do, but certainly they'd like to see the whole class in the least. They probably want something larger, at least 500 lines. Its hard do do anything really interesting in less than that.

With the latter, they usually want whole *systems* of code that solves a non-trivial problem. One such company I know of requests samples be around 5000 lines, and they also discourage submitting homegrown 3D math libraries and simple DirectX/OpenGL "wrapper" classes because they don't really demonstrate your problem solving ability. For these types of folks you probably want to send them a fully working sample to demonstrate some feature you've written -- perhaps car physics, particle systems, or cloth simulation.

Depends on which category the company in question falls into. If the former, pick maybe 3 largish, possibly-related classes that you contributed significantly to, and which you are most-proud of.

If the latter, send the complete system of classes, if not the entire application.

Remember that this is not only a showing of your skills, but hopefully is also an opportunity to talk about some of the cool code you've written during the interview -- Code with interesting bits to talk about therefore goes to the front of the line. In the same vein, be entirely clear up-front about what you wrote, and what other contributors wrote -- You don't want them calling you in thinking that someone else's code is yours.

Finally, you can probably ask someone what type/size/topic of samples they would prefer to see. Frankly they should have made that clear. But I'm sure they'll be happy to give you some guidance in the interest of getting the best sample to judge you by.