What is the diameter from the factory of the exhaust pipe attached to the exhaust manifold of a 1997 Ford F150 4.6L RWD automatic?

 

I believe by the manifold it is 2.25" but after the cats it is 2.5"

 

Today I was replacing the EGR tube and removed the inner fender well liner to get more space to work with. I measured the drivers side exhaust pipe just below the flange using a 6" digital caliper. The pipe measured 2.44".

 

Pretty sure pipe is measured from the inside diameter or I.D.
Most likely it is 2.25 id with 12 gauge pipe wall, which should bring it to near what you measured