function GetFileSize_a(FileName: string) : string;
var
H: THandle;
begin
H:=CreateFile(PChar(FileName), GENERIC_READ, FILE_SHARE_READ, nil, OPEN_EXISTING, FILE_ATTRIBUTE_READONLY, 0);
Result:=GetFileSize(H, @Result);
end;

if GetFileSize_a('1.exe') > [F] then
   ...