nsIFile:fileSize
From MDC
[edit] Summary
This attribute exposes the size of the file referenced by this nsIFile.
attribute PRInt64 fileSize;
[edit] Remarks
The value of this attribute is the number of bytes corresponding to the data represented by the file. Any meta data, such as a resource fork on the Mac, is not included in the measurement of the file size.
Changing the size of a nsIFile operates on the underlying filesystem, possibly truncating the existing file to specified size.