The cubic-to-rhombohedral (C-R) phase transition of the ferroelectric relaxor Pb(Zn1/3Nb2/3O3) is investigated by means of a high-resolution x-ray diffraction study on single crystals. The phase transition is diffuse and spreads over the temperature range TCR = 385 ± 5 K to TR = 325 ± 5 K. Below TCR, the cubic phase transforms progressively into rhombohedral domains whose average size, about 60-70 nm in the [111] direction, remains unchanged as temperature is lowered, so no growth of the R-domains is observed. At TR, the nanometric R-domains fully pave the crystal but structural mismatches between adjacent domains generate stresses, which increase as temperature is lowered. The anomalous broadening of diffraction peaks of the R-phase, which originates from size and strain effects, can be suppressed by applying a dc electric field, along the [111] direction, which transforms the polydomain state into a rhombohedral quasi-monodomain state.