How to define buffer size via parameter in auto generated C code?

Since I did not get an answer on my last question, I try it once again with a more detailed explanation:

I'm using a buffer from DSP System Toolbox and genereate C code with Embedded Coder. The buffer size is defined by a parameter "BUFFER_SIZE" of let's say 100. After generating auto code the parameter is disappeared and the "100" is hard coded. Since I would like to have the possibility to change the size of the buffer in the C file, the question is: how can I bring the code generator to keep the parameter "BUFFER_SIZE" as the definition of the buffer size so it can be changed in C code afterwards?