At what size will insurance allow single hand operation?

Depends on where you are and your experience.

Typically 70/75 is where most will start requiring a full time captain but there will be exceptions. The question is more like will you be willing to spend all the time required to clean, repair etc. yes you can hire day worker for a number of jobs but when an impeller goes on the generator in the middle of a 90 degree night or an air con pump goes kaboom in the middle of lunch...

It depends on the boat and on your personal standards. I d say one to two hours a day

Not much to add to previous - all valid - comments.
But I hope you don't mind if I challenge your question a bit, 'cause you seem to imply that you are interested in the largest boat that can reasonably be handled with no crew.
Now, possibly against the common thinking, my personal attitude towards boats is that you should look for the smaller size which is large enough for your needs, not the larger one that you can afford, or that the insurance allow you to handle, or whatever other reason where the focus is on "larger".
And the very simple reason is that keeping a boat afloat is hard work, bound to give headaches proportional with her size. To put it bluntly, why should you go for a big headache and suffer it yourself, if you could have someone else getting sick on your behalf?

BTW, I see that you are based in Finland. Apropos, welcome to the asylum.
It's always nice to have someone else challenging all these Americans who pretend that FL is a good place for boating...
Back to the point, by sheer coincidence, a couple of Finns friends of mine sprung to my mind when I read your OP, because they fit perfectly your description of a motivated family with kids old enough to be useful in managing a boat.
But theirs is a 43 footer, and their cruising experiences (which I had the privilege to share, in one occasion) are nothing short of epic, if you see what I mean...

"Operate" the thing is just the fun part.
If you're planning to have no crew at all, I'd be more concerned about the upkeeping part, i.e. everything the boat demands before and after operating her - and in many ways, regardless of how much she's actually used!

PS: just to put my comment in perspective, myself and my wife have been living aboard for anywhere between 5 and 7 months/year in the last decade, with zero crew.
But every time we simply wash our 56 footer, we regret not having something smaller.
And that's without even starting to consider more demanding maintenance!

50 60 70 or 80 feet makes little difference when it comes to handling or even single handling Regardless of size the captain needs to put the boat in a position where lines can be passed ashore. The size doesn’t make much of a difference. Whether 60 or 90, one is not going to manually wrestle the boat in position

What is far more critical to ease of handling is the boat layout. A boat with a pilot house on the main deck and doors on each side is probably the easiest to single or short hand as you are within a few steps of your spring lines. On the other hand a flybridge boat without controls in the cockpit will be the most difficult regardless of size

same with walk around decks. I d rather single hand the 84 MY I used to run than 60 footer with extended deckhouse and no side deck.

As to incapacitation, yes both husband and wife need to be able to take over if needed and at least get the boat out here help can be obtained. It s one thing for a spouse to be able to dock the boat while you re standing a few feet away. Now if you re dying on the pilot house floor ... the psychological pressures will be tremendous. My instructions to my wife are clear: call for help and get the boat outside whatever marina or harbor, and drop the hook.

but again, it s not about size it’s about layout.

It also depends on where you use the boat. For instance in the Bahamas pretty much every marina has a couple of guys ready to watch your boat as soon as you dock. Great for an owner operator couple. I use them sometimes so we can take better care of the guests.

same with interior cleaning... we ve never had issues finding a cleaning lady in the Bahamas when short on time to get the boat ready.

There is no doubt that a bigger boat cost more to maintain but it also also easier. I believe Brian has a 70 Johnson; I ran one for 8 years so I m intimately familiar with the engine room and the systems. It takes me half the time to do the same the maintenance tasks on the 116 Lazzara I run now. I no longer bang my head on the ceiling or have to crawl in a corner to reach a genny impeller or squeeze along the front of a hot engine to access a fresh water pump.

bigger boats also have redundant systems so often if something fails like an air con pump, genset, water pump etc you can just switch to the back up. As an owner operator you don’t have to abandon you family on the spot to fix something.

so many factors.

Ferrus:
My wife and I operate a 75 Hatteras MY without any other crew. I've had a number of other boats under 60' and had no problem insuring the 75.

While I enjoy hands on maintenance, I will hire out the task when time does not permit me to accomplish same. I DO NOT enjoy washing the boat, consequently, I hire a crew to wash every week or after use and try to pre-arrange the task when we cruise.

If we will be going a longer distance and need to crank out miles rather than enjoy the voyage, I hire a mate and or captain to assist with the trip.