We were able to sign up a few thousand people to our new product waitlist through two main channels: exhibiting at conferences and answering questions on Quora. In the past, I would have just done a small beta with friends, but I guess I'm getting a little wiser over the years :)
Going big from the beginning is really a tough one. I already got burned at a previous startup by not narrowing the focus enough, but it really depends on the market and the type of product you are building. We're building a product for college students, which is a big group, so it's a toss-up whether to go after a certain category of students or go after them all. We're casting a wide net for now with the idea that our product will resonate with certain subgroups which we can focus later.