Size.cs

public async Task<string> GetFileSize(StorageFile file)
        {
            var basicProperties = await file.GetBasicPropertiesAsync();
            var length = basicProperties.Size;
            return Calculatesize(length);
        }
        
        public string Calculatesize(double sizeInBytes)
        {
            const double terabyte = 1099511627776;
            const double gigabyte = 1073741824;
            const double megabyte = 1048576;
            const double kilobyte = 1024;

            string result;
            double theSize = 0;
            string units;

            if (sizeInBytes <= 0.1)
            {
                result = "0" + " " + "bytes";
                return result;
            }

            if (sizeInBytes >= terabyte)
            {
                theSize = sizeInBytes / terabyte;
                units = " TB";
            }
            else
            {
                if (sizeInBytes >= gigabyte)
                {
                    theSize = sizeInBytes / gigabyte;
                    units = " GB";
                }
                else
                {
                    if (sizeInBytes >= megabyte)
                    {
                        theSize = sizeInBytes / megabyte;
                        units = " MB";
                    }
                    else
                    {
                        if (sizeInBytes >= kilobyte)
                        {
                            theSize = sizeInBytes / kilobyte;
                            units = " KB";
                        }
                        else
                        {
                            theSize = sizeInBytes;
                            units = " bytes";
                        }
                    }
                }
            }

            if (units != "bytes")
            {
                result = theSize.ToString("0.00") + " " + units;
            }
            else
            {
                result = theSize.ToString("0.0") + " " + units;
            }
            return result;
        }