Input from Cruisers please.
As usual, Jeff makes some very good points. However, I have a unique perspective in that I have cruised in a 30 foot boat (12k lbs) with 4 (2 children) for three years, and now have a 45 foot boat.
As far as speed, yes it''s true that big boats are faster. But, time and again, I found our little boat keeping up with or even passing 37 foot boats (under sail - under power we were always slower). The reason, I believe, is that due to our smaller sails and easier handling, we usually had the correct amount of sail up, while bigger boats were worried about having to put the reef back in, or striking a big sail and therefore sailed less efficiently. I was a more relaxed sailor since I didn''t worry about sheet loads that were 4 times mine. A broken block on a 30-footer does not have the same damage/personal injury potential as on a 45-footer.
Jeff is right in that longer waterline, more sail area, etc... makes a faster boat, but dosen''t take into consideration the sailors themselves. I found this discrepancy common.
I once sailed from Panama to Honduras in company of 2 boats - a 48 footer and a 37 footer. The conditions were 30 knot winds and 8- 12 foot seas, near close reach. The first day, the big boat turned back because their auto pilot failed and it was too difficult to hand steer. It would have been hard, but I would have kept on - my boat was easier to steer. The 37-foot boat arrived 18 hours (over 600 miles) after we did. They said they reefed way down and had a slog. We got beat up pretty well, but sailed at near hull speed most of the way.
Now that I''ve got a bigger boat, though, I see the many advantages. I carry 3 times the water and food, and more spares. My boat is much faster (assuming I''m willing to push it), but most importantly, more roomy and comfortable. Our final trip in the smaller boat was through a very rough Gulf Stream. The trip soured our cruising for awhile. Wet sails were down below in the way, there was no comfortable place for the 3 off-watches to rest and the motion was unbelievable. I wonder sometimes how the trip would have been with 3 times the weight, 15 feet on length, and 1 or 2 more knots.
What this means is that a 31 foot boat is plenty big enough for 2 adults and 2 kids that will coastal cruise. Off-shore (say transatlantic) is dicey due mostly, in my mind, to the lack of tankage (maybe you can fit a water-maker, but it''s unlikely). Comfort is relative. If you want to cruise, my experience says keep it small and simple. If you want to be comfortable and go fast, and carry every spare (and be more of an object of scorn to the poor locals) get a big boat.