filesize -package:basement -package:ghc-lib -package:filemanip package:hackage-security

For some files we might not know the size beforehand, but we might be able to provide an upper bound (timestamp, root info)
For most files we download we know the exact size beforehand (because this information comes from the snapshot or delegated info)
For a handle hdl which attached to a physical file, hFileSize hdl returns the size of that file in 8-bit bytes.