Your cranking battery size should be determined by the engine manufacturer based on CCA. I’d say probably gp27 in your case.
Your decision to add another battery is another matter entirely. Provided that it’s a house battery, it’s size will be entirely based on the capacity you need. There’s no real “right” answer, it’ll be determined by your electrical load on the system.
If your concern is simply to have a backup battery in case of a failure of the one in the bilge, you can probably just use the same size you currently have, provided that you judge yourself to have adequate capacity as it is. You’ll have to be careful about how you wire that battery so that it’s a true “independent” battery, as opposed to simply part of a larger house bank, which would defeat the redundancy concept you seem to be after. However, this will also have implications as to how the batteries will charge from the engine alternator, especially if you’re using a BEP type switch.
Lots to consider here.