Dear Doaa,
the sequence length is chosen based on the modulation type and depending on the system testing conditions. So, for a 16QAM modulation you need to have enough symbols to get all 16 possibilities, so it is not enough to use 128 bits as a sequence length and so forth. You must start with proper length. Please refer to the settings in the examples published with OptiSystem software located at the following link.
Also, you need to be more realistic for random data transmission, which requires longer sequences. Typically, you start with low bits sequence then try to increase it the see if there is a large difference in performance. If there is you need to increase the sequence length.
C:\Users\USER NAME\Documents\OptiSystem 17.0 Samples\
Regards,
Ahmad