If you mean file.available(), SdFat does not provide available() since this function does not always provide the correct result in SD.h. The problem is available() is declared as an int in the Stream class. Type int is limited to 32767 bytes but files can be much larger, up to 4294967295 bytes.
You can use the following expression in SdFat:
bytesLeft = file.fileSize() - file.curPosition();
Too get the correct result with SD.h use this expression instead of available():
bytesLeft = file.size() - file.position();