Serial and IOCTL Driver serial_buffer_t size of txbuf

I am curious if there is a way to get the size of txbuf in the struct serial_buffer_t?
It doesn’t need to be at run time it can be at compile time.
Is there an API call or define that give this information?
I know the call tcgetbuffers returns the amount of data that is currently waiting to be transmitted in the txbuf.
There might be a very legitimate reason why this information is hidden in the API but I am not sure why?
Thanks in advance.

What Digi platform or product are you using?

are you using the AnsiC Library on windows?