Hi Kiwi
Don't want to sound "unhelpful", but there really is no way to answer that question with that little amount of data.
There is way more science in exhaust design and tuning than most people realize. Whoever designs your exhaust needs to know - - >
the state of engine tune,
rpm you want it optimized for,
length and diameter of the primary headers, (they alone could be far from optimum),
length and diameter of any intermediate piping,
back pressure in the intended 'muffler' (if any) at different volume flow rates,
etc etc
for street use, you can't go far wrong with sticking with the collector outlet dia of 2.5 inches for the remaining pipe;
I wouldn't think you should step back down to 2" after the 2.5 collector.
cheers