Impedance Matching of an Antenna
Question - How to find the input impedance of an Antenna of a
  particular length?


This is for a monopole but there are equivalent diagrams to be found for other antennas such as dipoles etc..



You've seen this before in a question I answered here.


  Is there any method and how to calculate the stub length? Is there any
  formula?


This is the formula that folk use: -




\$V_P\$ is velocity of propagation and is around 0.7c for your average coax cable.


Picture taken from my answer here.