What came stock and how is it?
My Malibu came with a 14X20 and it is ok, but could use a little boost when weighted so I tried a Cutter 14X18, what a POS. It felt like the trans was slipping BAD and it vibrated. Not near enough prop.
Sorry, I don't know what size you should get but I know what brand NOT to get.
Post you results, I am still thinking about trying another prop.