I do not think that a strom jib really makes sense on a boat the size of a Cape Dory 26 but storm jibs seem vary in size between 70% of the foretriangle down to as little as 40% of the foretriangle. They need to be very flat, and heavily constructed. Thier sheets are generally spliced directly onto the sail. They require large reinforcing patches. They generally have very high cut foots so that they don''t catch waves and are fairly high aspect ratio. They are often lead to heavily reinforced sheet lead locations.