We show that metastable rocksalt CdSe nanocrystals can persist at ambient pressure depending on the physical size of the particle. The size-dependence of the hysteresis loop was measured for the solid-solid transition in CdSc nanocrystals, between four- and six-coordinate structures. A systematic shift of the entire hysteresis loop to lower pressure results in a threshold size of similar to11 nm for ambient metastability of the six-coordinate rocksalt structure. Smaller nanocrystals transform back to the four-coordinate structure as occurs in the CdSe bulk solid. Surface energy contributions are used to explain the shift. The results have important implications for the optimum synthesis of metastable nanocrystal solids under ambient conditions.