Typically, a 30 amp breaker is used for the generator feed. The installation instructions for the interlock kit will dictate where the generator breaker must be located with respect to the main breaker. The kit must be installed exactly as shown in order for it to be effective.
Interlock kits may not be used interchangeably with different branded panels. In fact, the kit must match the brand of the particular panel that it will fit into. For example, a Square D interlock may only be installed in a Square D electric panel.