There are a lot of threads discussing this and it seems that they all come to about the same conclusions- 35-40' is about right for a liveaboard couple. It seems to me that it is also a matter of personal preference, some are perfectly happy and comfortable with a 35' and others need the space of a 44', or larger. There is no one single factor that determines this but rather a complex personal formula that involves the type of sailing anticipated, costs (purchase, maintenance, etc.), storage capacity, physical size and strength of the owners, and on and on.... I have concluded that taking all the factors into consideration that 38' is the minimum that I will be comfortable in and 41' is about as large as I would want to buy. This is based on decades of owning boats of different sizes, and I am only planning to live aboard from 3 to 5 months of the year. I just met a couple that had a 42' for many years and concluded that now that they are full time liveaboards that 35' is perfect, so they had a custom all aluminum cutter built and they couldn't be happier. As with most things there is no "formula" that works for everyone.
SV Laurie Anne
1988 Brewer 40 Pilothouse