This depends on your application. Teams have used either #25 or #35 with success (and rarely, other sizes as well), but there have been some ugly instances where the wrong chain for the job was used.
Estimate the loads on the chain by consulting a design manual like
this (look for ANSI roller chain, e.g. RS25 or RS35) to get an idea of the power that the chain will accept, at a given speed. (That manual lists the allowable horsepower for each chain type, for a given sprocket size, at some speed, assuming the full working load is applied. Note that the working load is significantly less than the breaking load, because the manufacturer has selected a factor of safety.) Then have a look at the
Tsubaki troubleshooting guide to avoid the situations that can damage chain or reduce performance.
As a general principle, the tensile load in a chain is inversely proportional to the sprocket diameter. Big sprockets are safer than small ones, if the chain is at risk of breaking.