I would say that any boat that you can cope inside of, with simple rig sturdy enough for blue water crossings, safety gear that will keep you alive WITHOUT your boat, and an emergency beacon, provisions, etc. If you can fit it all in a 24 ft boat with a full keel and you have the experience and know your vessel then you should be fine. Of course there is a lot more to it than that.
Read blogs of sailors who have made crossings and learn from their mistakes. Watch their YouTube channels. Read every book you can get your hands on. Research the track record of the boat you intend to buy as far as how her sister-ships performed in a variety of situations. When you buy, sail her first and see if you can handle her by yourself. If you can, divide the effort by half and then your wife or a weaker sailor can sail it if you are incapacitated.
After you by the boat, sail it an do shake-down cruises to get familiar with her. If you are planning a circumnavigation, know that you don't have to have your own boat to do it as you can crew for others that have boats that are already insured and equipped to do it--gives you more experience on the open water. But when you go out on the big blue ocean, understand that the dynamics of it are considerably more profound than being in an estuary or off the coast. The farther from land the smaller the island you become.
Common sense is the rule governed by Murphy's Law. Even the most seasoned sailor can run into a harrowing situation. Even the stupidest ape can cross the North Atlantic in winter without incident. In other words, chance has a way of dealing interesting cards, it is how you play them that counts. If you feel safe on your boat and that you are going to have enough fresh water on-board to quench your thirst, food to sustain you and your crew, and confidence to weather hurricane force storms when hoving too, then you will be able to stay out on the water for long passages and enjoy the experience.
If any of this makes you feel apprehensive, then you are not prepared and despite how well equipped or sound your hull is, the deciding factor in living on a boat of any size on the open ocean is going to be YOU.
In conclusion, the size of your boat should accommodate the provisions and safety gear.
Your rig should not be over engineered to the point of complexity. It should be simple with all your lines leading back to the cockpit.
You must have jacklines from bow to stern, port and starboard.
You should have the ability to generate electricity by no less then three sources or means to do so.
You must have communication gear that is waterproof/submersible.
You should have good reefing systems accessible from the cockpit--especially if you are a solo sailor or have inexperienced crew.
You must know how to properly hove too.
I also recommend a boat with tiller steering, autopilot (both windvane and electric if possible).
Keep spare parts on-board for your inboard/outboard or other systems that will sustain you--like a watermaker.
I am sure that there are lots of other things but one thing you should consider is a radio with the AIS functions and ability to connect to the web with email through GPS.
I hope I answered your question with little to no emotion. Too many people that claim their boats are better because they are bigger or smaller miss the point entirely as they get red in the face and flustered. I've heard this conversation turn into an argument way too many times! Though it helps to have a large vessel for stability in big seas, smaller vessels have been proven time and again for being more than adequate if they are provisioned and equipped appropriately.
The boat is only as safe as her skipper--ultimately. If the skipper has weak crew-members on-board that are unable to handle the boat, that is on the skipper and not the weakest sailor. Why do you suppose that the Coast Guard and the US Navy hold skippers of civilian boats accountable as well as their own? Liability falls on the captain of said vessel. My many years of being a US Navy sailor has taught me that training everyone on-board is required. Everyone on-board must be an expert.
And before ANY skipper unties the lines to set out on a voyage across big seas, he must have a survival suit on-board--period.
This post is not intended to scare you, or anyone that reads it, but to put out there the reality of how potentially dangerous cruising the blue open water is.
One final note, and this may be in FAVOR of larger vessels: You should take into consideration how comfortable you want to be on the vessel in bad weather. Depending on the ocean and time of year, you may not experience extraordinary weather conditions. The fact is, most places that people cruise don't see a lot of fowl weather, it ends up being less than ten percent of the total cruising time. Sailors that have cruised for decades will tell you this also, the spending time on the open water is mostly calmer conditions with occasionally spirited sailing. But the hard part is that less then ten percent of the time when the weather is violent, or you are sick from motion. The smaller boat is going to get tossed about much worse than the latter. If you cannot handle the violence of the seas at their near-worst, then a smaller vessel is definitely not going to be what you want. Not everyone is going to experience "The Perfect Storm."
I hope that I have near-completely answered your question, based on my own experience and research. I own a Nicholson 33 and she out-performs any vessel I have owned. She also comes from a great pedigree/class of vessels from one of the finest yacht builders in the world. Quality of a boat cannot be substituted. It's like a car: a 3000.00 USD vehicle is gong to have much different performance and amenities than a 30,000.00 USD vehicle. Same is said for boats.
Fair winds and following seas!
