The first task!!!
The first task of breakers and fuses is to protect wires and downline equipment.
In order to enlarge a fuse/breaker, you must inspect all the wiring and pieces of equipment to ensure it is listed/rated for that use. For instance, take an American panel with a 100A breaker feeding a subpanel whose buses are rated for 125A, fed by wire sized 1/0 AWG. You want a 125A breaker. That checks out, because the wire and subpanel can handle it, as can the wire. However you can't do a 150A breaker because the subpanel can't handle it.
Sizing the main when minimum sizing is valuable
I understand in Japan, you are flat-rate-per-month charged based on the size of your main fuse/breaker. You have incentive to specify the smallest possible. (honestly, anyone sizing an off-grid solar system has the same dilemma.)
To start with, you need to ascertain whether your service is 1-pole (100V) or 2-pole (200V).
In the case of 1-pole service, it is straightforward; you simply do the below calculations overall. However for 2-pole service, it is more complicated. You must ascertain which pole a circuit is on, and do the below calculations independently, for each pole. 200V loads count against both poles at once.
So, per pole:
You need to ascertain which circuits are on that pole (rather easy on a 1-pole service), then determine which appliances are on that circuit. Then, you list all the appliances on the circuit, and compute the practical ampacity of each of them.
Now, here's the tricky part: Group all the circuits into which pole they are in. Now, factor out appliances you don't expect to use *simultaneously** except, (the tricky part) do this across the entire pole, not per-circuit. Say you have a case where circuit 3 has a 13A washing machine. You never use it at the same time as a 10A bathroom heat-lamp on circuit 5. So you cross off the smaller load, and consider only the 13A load - even though they're not on the same circuit, they are on the same pole.
You obtain amps either from the appliance's nameplate amps, or from its "watts" or "VA" figure divided by the voltage - typically your voltage is 100V. However, sometimes, appliance ratings don't reflect actual draw - take a desktop PC. It might have an 850 watt power supply with a nameplate of 12 amps. However, in actual usage, it might only draw 3 amps. You can measure this with a device such as a "Kill-a-Watt", shown in another answer.
I actually recommend going to the extent of labeling circuits, so you can mark which breaker/outlet a load is on. Say, So for instance, since poles are so important here, circuits on pole 1 might get called Thor Hulk Ironman Hawkeye Cap etc. And pole 2 gets called Ariel Belle Cinderella Jasmine Mulan etc. Then turn each circuit off and go see which outlets went dead, and label all those "Thor" etc...
Balancing a 2-pole service
Now, you may discover that your pole loading is lopsided -- you may find your maximum simultaneous usage on pole 1 is 45 amps, and on pole 2 only 13 amps. Or you may discover the washing machine and bathroom heat lamp are on opposite poles, so you are unable to dismiss the heat lamp. It would improve balance if you could move things from one pole to another.
In that case, you (or your contractor) can do just that. However, you can only do this on a per-circuit basis, not a per-appliance basis. Hence, caring about circuit numbers/names.