Thats a difficult question.
-It depends what you mean by comfortable.
-It depends which direction the wind is blowing (Offshore 1 mile fetch, on shore, 100s of miles fetch so bigger waves).
-It depends how long its been blowing, and what the weather has been like in the preceeding days which affect the swell.
-It depends if the tide is with or against the wind.
-There is more to comfort than length.
I would use most 4m ribs in those conditions but the family would depend on the actual waves/weather on the day. You would be much more comfortable on the 5.4m than a 4m. The engine on the SR is not exactly this year's model - but that gives you scope for improvement.