Electric potential and length of wire?
If we use the above formula then it means that the voltage must
  decrease.


You're attempting to apply a result outside of the context in which it was derived.

Outside of a static spherically symmetric charge distribution with total charge \$Q\$, the electric potential is indeed given by the formula you quote.

But, in the circuit context you give, we're not outside of a static spherically symmetric charge distribution so this formula, and any conclusion you draw from it, isn't valid in this context.