Usually, a Letter in PRL is limited to 3750 words. Yet frequently papers from large collaborations at CERN papers exceed this limit by a lot. One example is the recent record-setting paper with 5154 authors: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.191803
But there are also examples in other fields, such as the first observation of gravitational waves: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102