Start with a requirements-based approach. What do you need and what is the minimum to achieve it.
Do you scuba dive? Do you need an on-board compressor? Do you work with your hands and need a certain amount of tools? What occupation will pay the bills and can it be done aboard? If so how much space is dedicated to it? I wouldn’t make a major compromise on the space needed for earning. It should not be shared with anything else that would cause you to take time out of your earning to neaten up or prep for earning, or that might preclude you from making a living on a given day because something else is taking up the space. Does your primary occupation require dressing a certain way? Living aboard is not a fancy, permanent press lifestyle. Does your primary occupation require being away from the boat for extended periods? Do you have someone to be present on board during those times or a means of securing it? How many people do you need berths for?
Next, you want to be able to make ocean passages. So you need certain minimum systems: water maker, cooking, bathing (may not be daily!), communications. How much space do these take up, with extra allowances for fuel, battery space, and redundancies? When you get to your overseas destination, what kind of stuff do you need with you? How much space does that take, including any maintenance function or to take care of equipment, clothing, or tools you need for that stuff?
Answering these kinds of questions will be better at estimating minimum necessity rather than starting from length and working backwards.