That's not a complete question (or at least not a correct one) since capacitors aren't sensitive to light. Could you show us what the circuit looks like?
I thought that under bright lighting conditions, a larger capacitor is needed because the LDR's values are pretty small, so the decay times are also small capacitior
You're correct. The Stamp measures the decay time and, for the best resolution, the capacitor value should be chosen for the expected lighting conditions. If the capacitor is too small, the decay time will be too short for the Stamp to measure with more than a few distinct values. If the capacitor is too large, the decay time will be longer than the longest time that the Stamp can measure.
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- Stephen