Hello Sahil,
The PRBS generator is used to produce bits that could represent actual data transmitted in real life. The longer the bit sequence the better. However, that causes limitation on the computer memory and speed of calculations. Typically, 2^16-1 is suitable for the modulation scheme you are using. In OptiSystem, we allow total bit sequence and samples per bit (equal to number of samples) to 2^31-1. so you can adjust both parameters to be within this limit.
Regards,
Ahmad