If going dual I'd go with 2". Single: 2.5"

Pi*r^2...

2" cross sectional area is 3.14". 2.5" is 4.9" A 2.5" on one of these inlines is too much if you're running dual pipes.

http://www.magnaflow.com/07techtips/faq/question10.asp

Handy chart for your use.