I really appreciate all of your thoughtful responses!

I didn't even know Cycle Gear existed, so I learned that today-- unfortunately the nearest one is 77 miles away.

I returned the helmet in question and ordered the next size up. If it arrives on time (Thursday) and if it fits, I'm good to go. If it doesn't fit, I may have to 1) try to make a trip to the Cycle Gear and try on some helmets or 2) buy one at this dealership and hope for the best.

Saturday is the big day, either way, and I'm looking forward to my first ride!