This was just a drawing I had kicking around so I edited it for illustration.

The garage circuit breaker would be sized according to the size of cable feeding the garage and several other factors including the installation method used for the cable and the length of the cable etc.

It could be okay to use the same size breaker for the garage as the main breaker if the cable size was sufficient but it would be good practice to introduce 'discrimination' by making the garage breaker a faster curve than the main breaker ie the garage breaker might be a 'B curve' and the main breaker could be a 'C curve'.
0600DB0105.pdf (first page for brief explanation)