How you select a circuitbreaker?

A circuit breaker is intended to protect the wiring of the circuit from overload.

The larger a CB is, the faster it will operate, thus in a short circuit situation, the (say) 60Amp breaker will trip before the 30 amp one that is in series.

You should only have a moderate factor of safety in your choice of CB.
The wiring itself will have a recommended current rating and this is a guide as to the CB.