The sub-panel in my detached-garage is a 4-fuse box that is supplied by two 10/2 wires with ground. The 10/2 feed wires are routed through underground plastic conduit. Each 10awg feeder wire is connected to separate 20amp breakers in the main 150 amp service panel. My goal is to upgrade to a larger subpanel in garage using the existing feed wires. (As pulling larger wire through the existing conduit is not an option.) I only need to add one or two 20amp circuits. I believe I'll need to change the two 20amp breakers in the main to a larger two pole breaker and that I'll need to install a ground rod for the garage service, as neutrals and grounds are currently going back to the main service.

Question 1) what size breaker should I install in the main to accommodate the
10awg wire without over doing it? Is 30amp the limit?

Question 2) what size subpanel is appropriate for a 10awg feed?

The sub-panel in my detached-garage is a 4-fuse box that is supplied by two 10/2 cables with ground. The 10/2 feed cables are routed through underground plastic conduit. Each 10awg feeder hot wire is connected to separate 20amp breakers in the main 150 amp service panel. My goal is to upgrade to a larger subpanel in garage using the existing feed cables. (As pulling larger wire through the existing conduit is not an option.) I only need to add one or two 20amp circuits. I believe I'll need to change the two 20amp breakers in the main to a larger two pole breaker and that I'll need to install a ground rod for the garage service, as neutrals and grounds are currently going back to the main service.

Question 1) what size breaker should I install in the main to accommodate the
10awg wire without over doing it? Is 30amp the limit?

Question 2) what size subpanel is appropriate for a 10awg feed?