I will try to be serious and avoid using bites per mouth. So this is about the gibibyte (GiB) which is 2^30 bytes and the gigabyte (GB) which is 10^9 bytes. See Definitions of the SI units: The binary prefixes.
Well each saved log character is a byte and 16GB are 17.179.869.184 bytes
SD card capacity is specified as decimal GB just like disk drives. 17.179.869.184 is 16 gibibytes.
A 16 GB card has about 16,000,000,000 bytes. I checked a 16 GB micros SD and it had 31,291,392 blocks (512 bytes/block) or 16,021,192,704 bytes, not 2^34 bytes.
None of the above matters much for the original question:
the sketch is just opening a file adding a new line and closing it again. the plan is to do this for a long period of time.
The maximum size of a FAT16/FAT32 file is 4,294,967,295 bytes (4 GiB - 1). So when the file reaches that size, an error should be returned.
SdFat has a bug and a corrupt file will result. I used a max size of 4 GiB which overflows the 32-bit file size.
I will fix the next version of SdFat but the old version of SdFat in SD.h will not get fixed.
It's unlikely programmers will ever use the NIST units:
Examples and comparisons with SI prefixes
one kibibit 1 Kibit = 2^10 bit = 1024 bit
one kilobit 1 kbit = 10^3 bit = 1000 bit
one mebibyte 1 MiB = 2^20 B = 1 048 576 B
one megabyte 1 MB = 10^6 B = 1 000 000 B
one gibibyte 1 GiB = 2^30 B = 1 073 741 824 B
one gigabyte 1 GB = 10^9 B = 1 000 000 000 B