Deepblue, you sound like an experienced boater, but since you're soliciting opinions, let me share mine based on my experience from small boats to living aboard previously on a 65 steel bluewater twin diesel:
"I am retiring this year and upgrading my boat to travel 9 months a year with the wife." Consider this full time living aboard. To give some scale factor, I prefer living below my means in a modest home, but believe that 50ft is the minimum to have a comfortable amount of space, elbow room and a machinery space that you don't have to crawl through/lean over.
"Plan includes both coast for a few years." Although technically following the coastline, consider this ocean-going work. There are long stretches of unprotected coastline where you'll be 25-35 miles offshore and being hit by weather. In heavy weather, bigger is better.
"the extra cost to run" You don't mention anything about budget to purchase, so I can only assume you can afford to buy either. Then why the comment about "extra cost to run?" The costs for insurance, moorage, bottom paint every three years, fuel, etc., are all rather easy to calculate. So forecasting OPEX it should be rather straightforward.
"... a 47 nordhavn which is a great boat or go for the 62 nordhavn which gives us more elbow room for live aboard" What we call the "size" is the cube of the length of a boat. So your 62 is roughly 2-1/2 times larger. That's a big delta. How do you and wife feel when you walk and visit both boats?
"I have 45 years of boating and 66 years old... I am concerned that it is too big for us to handle" That would depend heavily on both your health and fitness levels. Do you partake in cardio classes, are you weightlifters and physical? Modern technology like thrusters can give you a big edge of course, and they don't break down too often.
Unless you're relatively infirm, you should be able to manage it and in some ways the larger boat is easier to get around than the tight confines of the smaller one. Big boats dock easier IMO. One advice is to keep the boat simpler if possible. That's tough to do with a Nordie since they are loaded with systems, but for example, go with a single versus a twin. Less maintenance and a better engine room.
"... the problem of getting slips that big." That's a real issue. When I was in Southern California I faced that issue a lot, including trying to find 50amp dock circuits versus 30amps. Can you focus more on anchoring out rather than tying up? That will save you ten thousand dollars annually. Load up on solar - try to be independent of land.
BTW, there have been several previous threads on this subject over the years, so do a search for other input as well.