We studied the silicic acid uptake kinetics of the pennate diatom Cylindrotheca fusiformis grown under a wide range of iron concentrations (from Fe-limiting to Fe-sufficient conditions) to assess the effect of iron availability on diatom cell size, silicon content, and silicic acid uptake kinetic parameters. As the iron stress increased, the growth rate slowed, cell size decreased, and silicification increased. A series of Si kinetic uptake experiments (from Si-limiting to Si-sufficient conditions) performed at different iron concentrations demonstrated the extent of the colimitation domain, where the specific Si uptake rate (VSi) varied as a function of both silicic acid and Fe availability. A decrease in maximal specific uptake rate of silicic acid (VSi-max) under iron limitation was observed along with a decrease in half-saturation constant for silicic acid uptake (KSi). Because VSi-max and KSi vary in the same direction, the specific affinity for silicic acid does not change under iron stress. The variation in cell size is an acclimation to low nutrient concentrations. Our study shows that Si uptake kinetics parameters, especially VSi-max, are strongly related to cell size, which is itself constrained by the degree of iron limitation. Thus, the surface to volume ratio should be related to silicic acid flows in size-based biogeochemical models of planktonic ecosystems. Another way to describe the observed variations of Si uptake over the full range of Si and Fe concentrations tested in the present study would be to implicitly take into account the variations in cell size through a scaling of the maximal specific uptake rate (Vmax) by the iron-limitation term in a multiplicative manner.