Storm jib

As big as you can handle & as small as you need.

Surely it depends upon what sort of sailing do you do, winds anticipated & where?
You are unlikely to be going to windward in a Cat, or wanting a strong wind beam on with accompanying waves, so 'off wind' is perhaps the order of the day.

If it really is 'storm' conditions, you might be better with bare poles laying 'a hull' or with a sea anchor/drogue.
 
Try phoning Paul lees at Crusader, he has a sort of chart that he picks given your rig measurements, OA Length etc.
Mine for 35ft Mono is 70sq ft and fits well with the emergency forestay rig.
 
One of the problems with a storm jib in a real storm is that it invariably proves to be too big (Sidney/Hobart report).

Though I have a storm jib (60ft2) I prefer to use the working jib (127ft2) on the roller reefer.

I've sailed to windward on this rolled to about 30ft2 in about 50 knots and found it quite large enough, thanks.

The designer of the craft would be the best person to advise - I'd hesitate to comment as some multihulls have very exiguous mainsails, whilst others are sub 3/4 rigs.
 
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