Yeah, try it on when you get it. Online reviews aren't super-helpful WRT fit, so don't give that a moment's thought. This is the central reason why I advocate buying a lid in a local store, so they have a reason to continue stocking them and people can try them on. An online purchase is only useful if you don't have to return it. After the return shipping you'll be looking at the same cost you'd have spent locally.
Obviously that wasn't working for you because of the brands and models the locals carried, which is a common issue. Hopefully you chose a place that doesn't charge a restock fee, too. Try it on, if it doesn't fit make sure you know why it doesn't fit (too tight overall, forehead is pressed, cheeks hurt, etc). Be sure to wear it for ~10-15min if its a close call because the lining will form-fit over time. A headache is a bad sign
If it still doesn't fit, take you complaints to the phone with the online store's customer service and pray they are capable of making a good recommendation.
Helmet shopping can be tough, so be patient!
BTW, doesn't your MSF provide helmets for the course? I've taken MSF in three different places and they all provided lids.....