Well as they say you can never have too much flow, but in this case I think a 600gph is plenty. It really all depends on what size pump you are going to use. If your pump is more than 600gph then you need to go with a larger box. The overflow box does not control the flow rate, the return pump does. The overflow box just needs to be rated as high or higher than the return pump you are using at the height you are using it.
So, if your pump is rated at 700gph and you only have a 600gph overflow your main will overflow. If you have a 700 gph
pump and a 1200 overflow box your flow will not exceed 700 gph