How do resonance structures affect bond length?
The bond lengths in the resonance hybrid should be an average of the bond lengths in the separate contributors.
Consider for example the nitrite ion, NO₂⁻. It has two resonance contributors.
The resonance hybrid has two identical N-O bonds. They share a double bond and a single bond between them, so the average bond order is 1.5.
We would predict that the N-O bond length should be halfway between a single bond and a double bond.
The N-O single bond length is 136 pm. The N=O double bond length is 115 pm.
The average of these bond lengths is 126 pm.
The observed bond length in NO₂⁻ is 125 pm.