A char is by definition always 1 byte (even if it's not 8 bits), so to allocate a specific number of bytes, just multiply in the malloc argument.
char *bytes = (char*)malloc(136 * sizeof(char));
Be aware, that you could also use
calloc, which takes 2 arguments and gives you zero'd memory, which is often times more desirable.
char *bytes = (char*)calloc(136, sizeof(char));