There are 8 bytes available at each of the interrupt vectors but you are not limited to an 8 byte ISR. If more space is needed simply AJMP to another location in CODE memory. Just make sure that the ISR, wherever it ends up, ends with a RETI. It is not good practice to let your ISR continue into another interrupt vector location, even if unused. Instead, place a RETI at each unused vector so that should an unintended interrupt occur your code won't run off into the enchanted forest.

Hal