I've noticed this question had been asked multiple times, but I have a slightly different situation.

So my house (in Malaysia, 240v 60Hz) had a main circuit breaker with 30amp currently. The main breaker is then connected to a circuit breaker box with about 20 individual circuit breaker (some 10amp, some 20amp), which connects to different parts of the house.

Now the main circuit breaker was broken (aging, probably), and I was wanted to change a new breaker. 30amp is enough so far, but I've planned to install another air conditioner. I've made some calculation and found the current requirement will be slightly larger than 30amp in peak hours.

The electrician here suggested me to change it to a 60amp breaker (there're two types, 30 and 60 only). He claimed that the cable from input, to the main circuit breaker would be able to withstand 60amps. And since the circuit breaker box wasn't changed, the maximum current through each cable is still maintained - hence no fire hazard.

It sounds right for me, but that's just some random electrician which I'm not sure if I should trust fully.. What do you guys think?

Since we do not know the size of cables, amp rating of the circuit breaker box, or any other specific details, there is no possible way for any of us to provide accurate advice.

Leaves me no other choice but to recommend you hire a qualified electrician to come in and evaluate your system.

Ah all right, thank you!