The transient elongational viscosityηe(t)of the polymer melt is known to exhibit strain hardening, which depends on the strain rateε˙. This phenomenon was elucidated by the difference of chain stretching in the entanglement network between extension and shear. However, to date, the microscopic evolution of polymer melt has not been fully statistically analyzed. In this work, the radial size distributionsP(Rg,t) of linear polymers are explored by dissipative particle dynamics during the stretching processes. In uniaxial extensional flow, it is observed that the mean radius of gyrationR¯g(t)and standard deviationσ(t)remain unchanged until the onset of strain hardening, corresponding to linear viscoelasticity. BothR¯gandσrise rapidly in the non-linear regime, and bimodal size distribution can emerge. Moreover, the onset of strain hardening is found to be insensitive to the Hencky strain (ε˙Ht) and chain length (N).