This is a tough question to answer. The chlorophyll amount is highly variable, depending not only on the size but the physiological state of the cell. Photosynthetic pigments usually undergo degradation in response of (photo-)chemical and biological processes.
According to Stauber and Jeffrey (1988), chlorophyll (a+c) content ranges from 0.02 to >100 μg per million of cells. These values are from marine diatoms. Maybe you can get an average concentration from Thalassiosira spp. listed in table 7.