drain pipe does 600gph max. 1.25" ID
drain pipe does 900gph max. I believe that max rates can be achieved only with straight flow, with no elbows or bends in the pipe.
Here's a calc:
Go larger than what you need, and remember that drilled hole size needs to be 1/4" to 1/2" or more bigger than pipe ID
The standpipe pipe inside of the overflow can also be larger than your drain pipe ID
size, if you use a PVC
size adapter. My standpipes are 1/2" larger than my drain size.