Module ModFilesizeConversion
    Public Function ConvertSize(ByVal fileSize As Long) As String

        Dim sizeOfKB As Long = 1024              ' Actual size in bytes of 1KB
        Dim sizeOfMB As Long = 1048576           ' 1MB
        Dim sizeOfGB As Long = 1073741824        ' 1GB
        Dim sizeOfTB As Long = 1099511627776     ' 1TB
        Dim sizeofPB As Long = 1125899906842624  ' 1PB

        Dim tempFileSize As Double
        Dim tempFileSizeString As String

        Dim myArr() As Char = {CChar("0"), CChar(".")}  'Characters to strip off the end of our string after formating

        If fileSize < sizeOfKB Then 'Filesize is in Bytes
            tempFileSize = ConvertBytes(fileSize, convTo.B)
            If tempFileSize = -1 Then Return Nothing 'Invalid conversion attempted so exit
            tempFileSizeString = Format(fileSize, "Standard").TrimEnd(myArr) ' Strip the 0's and 1's off the end of the string
            Return Math.Round(tempFileSize) & " bytes" 'Return our converted value

        ElseIf fileSize >= sizeOfKB And fileSize < sizeOfMB Then 'Filesize is in Kilobytes
            tempFileSize = ConvertBytes(fileSize, convTo.KB)
            If tempFileSize = -1 Then Return Nothing 'Invalid conversion attempted so exit
            tempFileSizeString = Format(fileSize, "Standard").TrimEnd(myArr)
            Return Math.Round(tempFileSize) & " KB"

        ElseIf fileSize >= sizeOfMB And fileSize < sizeOfGB Then ' Filesize is in Megabytes
            tempFileSize = ConvertBytes(fileSize, convTo.MB)
            If tempFileSize = -1 Then Return Nothing 'Invalid conversion attempted so exit
            tempFileSizeString = Format(fileSize, "Standard").TrimEnd(myArr)
            Return Math.Round(tempFileSize, 1) & " MB"

        ElseIf fileSize >= sizeOfGB And fileSize < sizeOfTB Then 'Filesize is in Gigabytes
            tempFileSize = ConvertBytes(fileSize, convTo.GB)
            If tempFileSize = -1 Then Return Nothing
            tempFileSizeString = Format(fileSize, "Standard").TrimEnd(myArr)
            Return Math.Round(tempFileSize, 1) & " GB"

        ElseIf fileSize >= sizeOfTB And fileSize < sizeofPB Then 'Filesize is in Terabytes
            tempFileSize = ConvertBytes(fileSize, convTo.TB)
            If tempFileSize = -1 Then Return Nothing
            tempFileSizeString = Format(fileSize, "Standard").TrimEnd(myArr)
            Return Math.Round(tempFileSize, 1) & " TB"

            'Anything bigger than that is silly ;)

        Else

            Return Nothing 'Invalid filesize so return Nothing

        End If

    End Function

    Public Function ConvertBytes(ByVal bytes As Long, ByVal convertTo As convTo) As Double

        If convTo.IsDefined(GetType(convTo), convertTo) Then

            Return bytes / (1024 ^ convertTo)

        Else

            Return -1 'An invalid value was passed to this function so exit

        End If

    End Function

    Public Enum convTo
        B = 0
        KB = 1
        MB = 2
        GB = 3  'Enumerations for file size conversions
        TB = 4
        PB = 5
        EB = 6
        ZI = 7
        YI = 8
    End Enum
End Module

This is module for converting filesize, round to highest unit : G, Mb, Kb, B

Filesize Converter

VB.NET