Excel - Concatenar texto con distinto tamaño de letra

Option Explicit

Sub Macro1()

    Dim a As Long, L1 As Integer, L2 As Integer

    For a = 1 To 100

        Cells(a, "C") = Cells(a, "A") & Cells(a, "B")

        L1 = Len(Cells(a, "A"))

        L2 = Len(Cells(a, "B"))

        Range("C" & a).Select

        With ActiveCell.Characters(Start:=1, Length:=L1).Font

            .Name = "Arial"

            .FontStyle = "Normal"

            .Size = 16

            .Strikethrough = False

            .Superscript = False

            .Subscript = False

            .OutlineFont = False

            .Shadow = False

            .Underline = xlUnderlineStyleNone

            .ColorIndex = xlAutomatic

            .TintAndShade = 0

            .ThemeFont = xlThemeFontNone

        End With

        With ActiveCell.Characters(Start:=L1 + 1, Length:=L2).Font

            .Name = "Arial"

            .FontStyle = "Normal"

            .Size = 10

            .Strikethrough = False

            .Superscript = False

            .Subscript = False

            .OutlineFont = False

            .Shadow = False

            .Underline = xlUnderlineStyleNone

            .ColorIndex = xlAutomatic

            .TintAndShade = 0

            .ThemeFont = xlThemeFontNone

        End With

    Next

End Sub