Recent work has made progress in relating the size of stomata to stomatal functioning and, in particular, the speed of opening and closing and its implications. Calculations of the influence of stomatal size on the potential rate of osmolarity increase, assuming size-independent ion influx rate per unit area of guard cell plasmalemma set at the value found in large (60 μm long) stomata, show that 10 μm long stomata could have at least a 6-fold higher rate of osmolarity increase. There could be a corresponding decrease in the time taken in going from the closed to the fully open state from about 1h to about 10 min; this is approximately the range found for stomata.. However, there are no data on the rate of stomatal movement over a sufficient size range to test this suggestion. Faster opening requires, assuming optimal allocation, a higher activity of the required enzymes per unit volume of guard cells. This is explored for cytosolic carbonic anhydrase which is needed in guard cells, at least in the light, for malic acid synthesis which is involved in stomatal opening in most stomata. Faster opening and closing of smaller than of larger stomata could allow closer tracking of environmental (mainly light) variations, although the available data are not adequate to determine if such a greater tracking occurs. The range of speeds of stomatal movement is similar to that for photoinhibition-related phenomena, despite the very different mechanisms involved.