What size downpipe ?

first of all my car is a BMW 530D

3.0 Turbo Diesel i6 engine

im looking into getting a downpipe fabricated up and im not sure what size i should be going for, in my head 3" makes sense but is it sensible to go bigger?, my power goals are 300-350hp

(i know im not running a VAG but this is the only place ive found where people know what theyre talking about when it comes to how diesel's work.)

What is the stock power? What is the stock exhaust? Hard to honestly say how large you would need. 3" will probably be a nice upgrade, but it's hard to speculate with little information.

sorry stock power is 220 and the stock downpipe is 2.25"

I would go 3" for sure. No point in upgrading .25"