In high-brightness electron linear accelerators (LINACs), the particle bunch length is measured by a radio frequency deflector (RFD). The electron bunch is deflected vertically toward a screen and its length can be obtained using vertical spot size measurements after a proper calibration, e.g., measuring the vertical bunch centroid while varying the deflecting voltage phase. The energy parameters of the bunch (the energy chirp and the energy spread) and the correlation between particle positions, divergences, and energies contribute to the bunch vertical dimension at the screen position after the RFD and so far were considered as a source of systematic errors in a bunch length measurement. The measurement theory and production model for bunch length, energy spread and chirp, as well as correlations are described. As usual in particle accelerators physics, the method is validated using numerical simulations of state-of-the-art LINACs with a reference simulation code showing a typical accuracy in the few percent levels.