I have 2&1/2" attached to the end of the twin engine pipes. This is bigger in area than the engine pipes (2 x 1&3/4"). The mufflers I use are straight thur with drilled holes & are 2&1/2" ID. There is no loss of bottom end torque with this setup on a mild B20B motor, but it is noisy. :) 2&1/4" is probably a more sensible size for a mild B20.

You can have oversize header pipes however. 2" x 26" long 4-1's would be useless compared to something more sensible like 1&5/8" x 32" long 4-1's but they probably still wouldn't hurt bottom end torque when compared to the standard cast iron manifold & dual engine pipe.