Capacity is the amount of power that you will need to run the electrical things in a house. The entrance (junction box between the meter and your house wiring) and the distribution panel (breaker panel) are generally sized by the electrician who will do the installation. If you have an electric stove, an electric refrigerator, a freezer (to store lots of food), and a submersible pump in a drilled well, lights, etc, these all have to taken into account, and the distribution panel in the house will have to be capable of handling these loads safely. Things like electric range/oven, well pump, clothes dryer are generally wired for 220V nominal because they are big loads and it's best to have have them fed from both legs of the breaker panel so the load is distributed across both legs instead of trying to draw a lot of power from one leg.
In Maine (at least if you are served by CMP/FPL) if you want to build far away from existing power lines the power company will mount a step-down transformer on the nearest utility pole and they will run a 220V line to the meter mounted on your house (or on a temporary structure built for that purpose) and YOU have to pay for every pole that they have to install between the transformer and the meter. If you choose underground service, YOU pay for all the trenching and filling and the direct burial entrance cable (which is more expensive than the overhead cable). The electric utility may not make you pay up-front for the poles, but if they don't, they will make you sign a service contract and every month, some of your utility bill charges will come from payments on those poles. If you discontinue electrical service, they will expect you to pay for the poles in a lump sum.
Now, about the logic and legalities of building a one-room cabin. Many municipalities will not allow you to reside permanently in any structure that does not have running water, heat, and sanitary facilities. Even if you can meet those requirements, you will probably have to pay for the entire project out of your own pocket. In general, banks will not loan money for the construction of a residence that does not meet FHA guidelines, or even more stringent guidelines. Banks often do not hold the loans that they make - they sell them to holding companies who may or may not bundle and re-sell them to investors. At every step along the way, the lending institutions, investors, etc want assurances that the mortgage they are holding is sound and is backed by salable property as collateral. You will find very few investors who will regard a one-room cabin in the woods a "salable" residential property. For this reason, if you want to build and live in such a property, you will have to pay for it out of pocket. Remember, too, that each municipality/township/county can have its own regulations regarding what constitutes a permanent residence, and you will have to pay to comply with those regulations. You can't just buy some land, throw up a building willy-nilly, and move in. If you do that, the next time you open your door, you will be looking at a handful of code-enforcement officers. They will ask to see your permits, and inspect the parts of the building that they are responsible for - structure, electricity, plumbing, sanitation, etc. If you did any of the work yourself, they will ask for written proof that a licensed professional inspected your work and signed off on it. If you don't have the paperwork in line or have violated codes, you are subject to fines and penalties until the facilities comply with the codes, and they may be cumulative ($X/day until the work is brought up to code and re-inspected).