Maximum stomatal conductance to water vapour and CO2 (gwmax, gcmax, respectively), which are set at the time of leaf maturity, are determined predominantly by stomatal size (S) and density (D). In theory, many combinations of S and D yield the same gwmax and gcmax, so there is no inherent correlation between S and D, or between S, D and maximum stomatal conductance. However, using basic equations for gas diffusion through stomata of different sizes, we show that a negative correlation between S and D offers several advantages, including plasticity in gwmax and gcmax with minimal change in epidermal area allocation to stomata. Examination of the relationship between S and D in Eucalyptus globulus seedlings and coppice shoots growing in the field under high and low rainfall revealed a strong negative relationship between S and D, whereby S decreased with increasing D according to a negative power function. The results provide evidence that plasticity in maximum stomatal conductance may be constrained by a negative S versus D relationship, with higher maximum stomatal conductance characterized by smaller S and higher D, and a tendency to minimize change in epidermal space allocation to stomata as S and D vary.