Congrats on your new bike! There's nothing better than being excited about a new bike and all the fun things you can do to it/with it/on it!

I'm not sure what you mean by "sizing a Brooks"? Do you mean selecting which model you want?

Also, be careful of leaving your steerer tube too long. I don't know what the vertical rise is that you're riding with, but if you ever get into an accident that pitches you forward, the steerer tube is in exactly the right spot to punch a hole through your chest. That's typically why shops like to cut them down ASAP.