I have owned a 34 footer for over 15 years and done deliveries on boats up to 44 feet offshore/coastal for several decades. I single hand a lot.
I would add a "plus one" to the comment by RichH.
About 40 feet, give or take, is a good max for a couple. Your age is significant, but not as important as your health and general physical condition.
Speaking very much "in general" and based on some cruising couples I know that are out there (Pacific) right now, I would say that around age 70 to 75 is close to a limit for this, no matter how good your dna.. altho we do have some friends cruising Mexico now on a superbly-restored Cascade 36, and they are a bit over 75.....
The only larger boats I have been around that would be OK for a couple are lighter boats that have a sail area more like a 40.... like a Santa Cruz 52/53 cruiser. No sailing time on one, I hasten to add. ( But, I
wish! )
If you are pleased & comfortable with your N-28, you should bear in mind that a lot of the current large production sail boats are
not designed primarily for... wait for it... "sailing" like your present C&C design. You have a superb boat now.
In an turbulent and indifferent ocean, filled with endless breaking waves for a day(s) at a time... you need to be on a boat designed "from the outside in" and not the other way around. One of the best deliveries (gale conditions for 36 hours) I've had was on a KP-44, and that
design heritage was the reason.
As to the other observations about ease of handling as boats get a little larger, i would agree. While I will never likely have another boat as much
fun to sail as our previous Niagara 26, it is indeed easier to move around the decks on our present 11k displ. sloop and it can be sailed reasonably easily single or short handed.
While you may find that most "power assist options" work well enough, remember how many
decades it has taken roller furling to reach the point where it is useful for extended off shore trips. Now imagine how long it will take main sail furling to slowly transition to that point. I know one rigger that commissioned that option on new Hu___ers, and said he would never recommend it to a friend.
Further, note that even "settled" technology like electric winches and windlasses are still constantly prone to the everyday failures that happen when salt water or vapor gets past their sealing systems.
Seems like we all Want More, but beware the ancient curse of "getting exactly what you want."
L