Great Planes's is 68 1/4 and is possible to measure as 70 depending how you do it.
Aero-Works had a 70 inch span but their site appears to be down...
There have been others I am sure...
You will note that there are several variants of the Pitts, some with the same wing top and bottom, others with a smaller straight wing on the bottom and a larger swept wing on top... rounded tips or square tips, and various cowls on the nose.
So its not unreasonable to build a lower wing identical to the top wing with or without ailerons. You may not match any full scale but it would take a Pitts expert to be able to say for sure.
Ensure the lower wing installs at +1 deg incidence relative to the upper (assuming the upper wing incidence locks in correctly with the center mount used on your plane)
CG is more dependent on the upper wing than the lower. You can normally have a flyable biplane just ignoring that you have the lower wing and setting initial balance point at 30% MAC of the upper wing.
Expect the plane to be very responsive... Definitely not for a beginner.