There is no mention of proportionality in Article 51but anyway, if you apply the article 51 of the UN chart, then you have to apply the things that go with this article => proportionality
Legitimate defense was originally customary, but is now codified by the UN charter. The UN Charter does not limit it.The book I showed you says that legitime defense is an unwritten (customary????) right and that the UN chart does not limit it. However, "the legality of legitime defense depends on the proportionality of the acts taken in its name".
As for proportionality, that is 1. open to interpretation, and 2. the opinion of a limited number of scholars. There is nothing either written or in general state practice that confirms this.