The Yamaha 9.9 DOES come with a 25" leg. It is the overwhelming choice among catamarans. But the most important part of the Yamaha High Thrust model is the high torque produced by the low gear ratio. This allows the use of a much larger prop, almost 12" dia. This size prop is close to what an inboard engine might use. As far as I know, Yamaha is the only one with these features, which are ideal for sailboats.
As far as your idleing problem, remove the carb(easy), disassemble completely,(no shortcuts) spray carb cleaner through every little hole you can find. I just did this to mine, and they're purring like new.