HOw to find the size of a file in SD card

I am using sparkfun can_bus shield which has SD card ,I am able to store the data and I need to know the size of the file.

File myFile; uint32_t amount=myFile.size(); Serial.println(amount);

when I use this code It returns 0, can somebody help me..........

How does your code actually know which file you are looking at?