using the formula in #2 is interesting but fraught with difficulty. With a pair of insulated wires, the 'effective permittivity' is a mixture of air and dielectric, and also the effective 'earth radius' is located at infinity.
So, how curtis is dealing with these uncertainties is not clear.
In post #4, you said you were working in a confined space so the basic formula will be incorrect because the effective screen diameter is not at infinity.
In post#5 I asked about the actual arrangement of your situation, because it is possible that a single coaxial arrangement will be more suitable. I cannot understand why, for the information you require, you need to have a pair of conductors. If you wish to measure capacitance of the pair arrangement, you will need a balanced bridge.
At some point you will need to make a capacitance measurement and this part of the problem may defeat you.
It is for all these reasons that I asked for a bit more info.