We report on the realization of large assembled arrays of more than 300 single $^{87}$Rb atoms trapped in optical tweezers in a cryogenic environment at $\sim4$~K. For arrays with $N_{\rm a}=324$ atoms, the assembly process results in defect-free arrays in $\sim37\%$ of the realizations. To achieve this high assembling efficiency, we equalize the loading probability of the traps within the array using a closed-loop optimization of the power of each optical tweezers, based on the analysis of the fluorescence time-traces of atoms loaded in the traps.

5 pages, 5 figures