DYI Standpipe

[Magma]

so i have an internal overflow on my tank with a 1.5" drain hole on the bottom of the tank, then goes into a ball valve to control flow into the sump. problem is its louder than all hell, so i looked into ways to silence it and seems like a stand pipe is the way to go.

my question is since its only a 35gal tank with a 1.5" drain hole should i keep the stand pipe at 1.5" or should i go bigger, or smaller?

keep in mind the overflow chamber is about 18" high and 3.5"x3.75" wide...