I think I would like a 40' sailboat with a room fwd and one aft and two heads. I suppose I could get along without a second room aft and a second head but I like room to grow. I just realized in my thinking of size that if I hold out my hands that distance (about 3 feet) would be all I would be missing to instead get a 37' boat.
Do you miss 3 feet of length that much on a sailboat? What length boat is enough for you? I hope nobody replies saying a 15' dingy is all they need to live on
The answer to that question today depends a great deal on the age of the boat. There are older 40 footers out there with less "space" than a modern 30 footer. But as a liveaboard it's the storage than comes into play, and as Idiens points out the new "spacious" boats generally are seriously lacking in that area. They've spread the accomodations out so far there's no room for lockers outboard or elsewhere, and all the under bunk space is for tankage.
Also some of the new boats are so spacious that proceeding the length of the cabin at 30 degrees heel risks a 8 - 10 foot tumble if you lose your grip on one of the few handholds along the way. (but perhaps that's not an issue with an at-the-dock liveaboard)
PBeezer seems to be quite comfortable living and cruising on a 32 footer - but often that comes down to individual personalities and their ideas of necessities and requirements. Find the compromise of space vs storage vs managability that works for you.