A bit of a guess....but I think it's 2.25" from cats to mufflers and 2" out the exhaust....behind the mufflers.

KW

 

I chopped up my pipes before the mufflers Saturday and they're 2.25". Anything after that I can't speak for. Te 2.25" H-pipe should be fine.

 

Yep, KW is right. It's 2.25" back to the mufflers and then 2" from after them on back.

 

Here's the Walker Exhaust catalog diagram for the '94-'96 LT1 B-body...
http://www.walkerexhaust.com/images/coupons/0263.gif

You can deduce the pipe diameters from the clamp sizes they call out.

The stock pipes are ridiculously small diameter. I can only imagine GM was trying to limit the engine power output for some reason.