What size prop to use?

[KQQLCAT]

I have installed a tachometer on my 20' pontoon boat becuase I thought that my motor was not reving high enough. I have a 2005 Mercury 60HP 4 stroke bigfoot and now I know it's doing 4,000 rpm's at full throttle at about 19-20mph.
I have a 15 pitch prop on there now. What should I go to.
Thanks

[boats.net]

in theory every 1 pitch down will raise your rpm's 200 and visa-versa.

OEM Mercury Propellers and Aftermarket Solas Props - 60 Sea Pro Bigfoot (15 Spline) 1995 - Present