RF Design - Antenna Feed-line Parameters
None of the calculations I've come across seem to care about the
  length


As with any high-frequency transmission line, the matching impedance is determined by the inductance per unit length divided by the capacitance per unit length and then by taking the square root of the ratio.

The division of L/m by C/m cancels out the "per metre" part and effectively, the impedance is unaffected by the length of the transmission line.