Answer:
Will be doubled.
Explanation:
For a capacitor of parallel plates of area A, separated by a distance d, such that the charges in the plates are Q and -Q, the capacitance is written as:

where e₀ is a constant, the electric permittivity.
Now we can isolate V, the potential difference between the plates as:

Now, notice that the separation between the plates is in the numerator.
Thus, if we double the distance we will get a new potential difference V', such that:

So, if we double the distance between the plates, the potential difference will also be doubled.
-- If velocity is constant, then there is no net force
on the chair.
-- If there is no net force on the chair, then friction
must exactly balance out your push.
-- The force of friction is exactly equal in magnitude
to your push, and in exactly the opposite direction.