I think 2.5 inch will be fine.
What are the sizes of the inlet/outlets?
If you have short i/c pipes then 3" will be fine, also I assume you have a 70mm external sized s13 throttle body, which will be better with 3".
Horses for courses, but if your i/c only has 2.5" inlet/outlet it wont make much odds.
If you have long 3" pipes then you would notice the extra lag associated with having to compress a higher volume of air
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