As I understand, the baby's size should measure from the date of ER. But just ask the dr to be sure. Also, when you have your first u/s they'll give you a due date based on the size at that point in time. My little one measured 2 days smaller than what he/she would've based on my ER date so my due date is 2 days later than the one I initially calculated based on ER. At my clinic a couple of days either way didn't seem to matter but each clinic will have their own criteria.
As for telling people, we guarded the fact that we were having IVF carefully when we were going through it. Only a handful of people knew. We didn't want the added pressure of people asking all the time if we were pregnant. The few people who knew we were doing IVF found out as soon as we took the HPT. Others found out after the 12 week u/s and bloodwork showed a healthy baby.
Now that we're safely pregnant, I've been very honest with everyone about having done IVF b/c I remember how hard it was not knowing anyone else who had gone through it. One friend who found out we did IVF has contacted me to discuss her own fertility probs and I've been so happy to be useful to her. Some people have actually said "what's IVF?"

But, as others have said, it's a personal decision and completely up to you whether or not you want to tell anyone how you became pregnant. It's nobody's business so you have to do what feels right to you.
Congratulations!
