My husband and I are planning on beginning the Atkins 20 plan this week when we return from a short trip. We are both in our 70s. I'm confused about the serving size of meat, fish, poultry, eggs. Also, how much can we eat of those items at one meal?

Welcome Suz,
Follow the "real food" plan in whatever Atkins book you have. The 1992 is summarized right at the top of the Atkins forum, that is a good one: http://forum.lowcarber.org/showthread.php?t=236482

I personally would not suggest using Atkins products.
There isn't a number for protein servings, but eating too much can slow weight loss. A general recommendation is a "Palm-sized" serving portion per meal, which might be 80g for a woman up to 120 g for a man total for the day. The 2010 Atkins book had some guidelines in it by height and gender.
All the best,