YC funds startups. Startups to us are companies designed to grow fast. Read this if you haven't: http://www.paulgraham.com/growth.html

It's OK to be working on something that's hard to imagine how it would grow to be a really big thing, that's how most ideas start out. (Apple made hobbyist computers, Google was another search engine of 1000 -- how big of a deal could they have thought these would become?). It's important to be open-minded to the pathways of how your idea could become something big as it keeps growing though, and it's good to stop and think about those ideas every once in awhile.

One of the biggest things I've learned from being around YC community is that most successful ideas start out as small but end up being very large. I would encourage you to think bigger about the fundamental problems you're solving and then how you can turn solving the immediate problem into something really big down the line. Hope this helps!

bootstrap. Then worry about VC money if and when you think you are ready to try to become a billion dollar company

10-20M sounds like a realistic projection for Dropbox. But they grew a little more than that.