It's important to dream. Stretch yourself and your vision to imagine the ideal library for your community. It will be unique and will contain everything you can possibly think of to make your library wonderful.
Then, come back to earth.
You have to be realistic. If you dream too big, your town may not support the project. It's essential to strike a balance between what you want, what you need and what you can afford.
If you plan a building that is significantly larger than the existing one, be prepared to defend the size with facts and statistics. That way you'll always have an answer to the questions you will certainly get.
If there's a unique feature that you feel strongly should be incorporated, cultivate donors to raise funds that are specifically earmarked to pay for it.
Make sure all your plans and ideas come with clear prioritization -- it's unlikely you'll get everything you want, and when the day comes to start cutting, you want to know in advance what you're willing to give up and what are absolutes.