Photodiode Trans impedance amplifier design
The photocurrent is determined by the amount of light falling on the photodiode. You can use other parameters such as the Responsivity to calculate the amount of expected photocurrent.

$$R(\lambda) = \frac{I_{phot} (A)}{P_{opt} (W)}$$

This responsivity changes depending on the wavelength of the light. In general, knowing the spectral components of the light source \$P_{opt}(\lambda)\$ you can calculate the total photocurrent using

$$I_{phot,tot} = \int_0^{+\infty} R(\lambda)P_{opt}(\lambda)d\lambda$$

Of course, this is a kind of annoying expression. Depending on the light source, you can probably approximate above expression with a much more simple expression. For LED's for example, you can assume all spectral power to be at a specific wavelength \$\lambda_{LED}\$, allowing you to just use the simple expression

$$I_{phot,tot} \approx R(\lambda_{LED})\cdot P_{LED}$$