Hi Francisco,
Since Bit error rate is the number of bit errors divided by the total number of transmitted bits that means BER is a ratio. Because of this we should not expect a different BER rate to come as a result of changing the sequence length. In general, changing your sequence length or making it longer will not change your BER if there is enough bits transmitted to characterize the system. Advanced modulation schemes requires long sequence lengths. If you do use short sequence length will get different results every time. However, when you use suitable length, the results will not change beyond that length.
I’ve attached 2 pics of the same system with the sequence lengths doubled in the second pic, the BER however remains more or less the same. (some variance due to random noise etc). To measure if your BER is 10^-9 or lower just run the project and the BER will be visible in the project browser. If you need a BER rate of 10^-9 or lower you will have to change other parameters and settings, but sequence length should not influence BER.
Best,
Josh