Warning: marinas are the world's most lonely places in the winter.
Ok, time for me to pipe in!
I lived aboard a Cal 25 for a summer, and a Catalina 36 for several years, here in Canada (and you KNOW it's the Land of Ice and Snow...). So concerning insulation and such: Not as much of a deal as you'd think. First, the ocean doesn't get that cold - so around 4 deg C, it starts getting WARMER near the water. Also, a layer of snow is insulation. I didn't have any insulation on my boat except on the mast - it's a BIG heat-sink! (and also usually a source of leaks when it rains).
I had "central heating": A Volvo forced-air unit that was crap - but look into Wabaso and even hot-water-heating systems. It's not hard to keep a boat warm in the winter. The other concern is of course sewage - if you have a holding tank it will have to be BIG and pumped out periodically. If not, you need a sewage hookup from the marina.
Which brings me to why I quit living aboard: here in Vancouver it's IMPOSSIBLE to find liveaboard space in a marina. The best you can do is find a marina willing to look the other way. Be sure to check out liveaboard space before you commit.
As for size, I had GOBS of space on the Catalina! Of course, it's a big 36, but I see no problem living on pretty much anything over 30 ft. My neighbours were mosly in smaller boats than mine: one was on a Cat 27 (although he'd tarped up the cockpit to make more room), another was a couple living on a 24ft powerboat.
There are some compromises that you will have to endure no matter how big the boat is: You need to go to a Rec Center for a decent shower every once in a while. You need to take you laundry to a laundrimat. (doesn't matter if you have a shower and laundry on board - trust me!) You will need a storage locker for... "stuff" that you just won't put on the boat. You'll need to arrange for phone, cable, internet... all the stuff you take for granted in a house.
But to (finally!) get to the post I'm replying to: I found more cameraderie and neighbours on the docks than I've EVER found in a house or appt! You really get to know your neighbours, help each other - there's a great sense of community on a liveaboard dock!
druid