so when there are resonance structures, all bond lengths are the same? Since I remember that double bonds are a little shorter than single bonds
No. In this case, they are the same because they are all equivalent; that is, they have the same connectivity (Cl’s attached to same Oxygen). However, not all resonance structures are equivalent for every molecule. Sometimes the traveling lone pair of electrons will change from being between an oxygen-carbon bond and a carbon-carbon (this is just another example, not the only example). However, in this case, one is more favored then the other because in this case one will be “more stable” then the other resonance structure, and in turn it will have more character of the more stable resonance form.
And be sure you know we are only talking about the bond lengths of the structures that contain the "resonance bond" for lack of a better term.