prop size

[indianabasser]

newbie here with a question. first off id like to say hello to all the members. i recently bought a 99 stratos 273 with a johnson 115 on it. im running wot at 6250 rpm. looked at the prop and its a viper ss 13 7/8 x 17. i found an omc 387520 14 1/4x 21 ss for $45. i think the size increase will get my rpms where they need to be but not sure if its the right size or do i need to be bigger. no jackplate. thanks in advance!!