At what size will insurance allow single hand operation?

Your questions indicate that you don't know what you don't know. You're asking about things that take a lot of time to learn. And Pascal's "two hours" are very optimistic in my eyes. If you are trying to keep a large yacht presentable with one or two people you are talking full time job. Yes, while cruising and enjoying but still you will be heading for that tool box all hours of the day. And washing, and and washing again. As far as boat handling...... a 50 or a 60 or a 70 is all very similar. But first you have to handle a 15, or a 24 or a 32........

No, but if it's a Princess you better bring a large tool box and you'll need to carry lots of parts. One should NEVER run a yacht by themselves. Unless you are SUPER experienced and have a yacht controller, you need a mate to get lines, fenders etc.

2 of us used to move a 220 footer around, no problem. We were young, fit lads and had our own hand signals after many years as kids. Not Rocket Science.

Single handing a 70+ footer can be done but not recommended, much easier and safer to have someone helping. As far as being a owner operator on a 70+ it is a daily job if doing the routine maintenance. Awhile back there was similar thread and Capt J described it as buying yourself a job, he is right. There is some satisfaction of being able to accomplish that, but it will turn into an job, especially if you want to participate in the fun stuff. As you said be prepared when taking on a 70+footer.