The bond length of a C=C double bond is approximately:
The average bond length of a C=C double bond, as in ethene, is ~134 pm due to increased electron density from two shared pairs, shorter than a C-C single bond (~154 pm) but longer than a C#C triple bond (~120 pm).
Ref: NCERT Class 11 Chemistry > Chapter 4: Chemical Bonding and Molecular Structure > Topic: Ionic Bonding - Lattice Enthalpy and Properties of Ionic Compounds