Word Font Size 12

Add the font size setting to the same innerRng.Font block where bold and color are applied. For example:

Sub Left()

    Dim rng As Range
    Dim innerRng As Range

    Set rng = ActiveDocument.Content

    With rng.Find
        .ClearFormatting
        .Text = "L\([0-9]{1,}-[0-9]{1,}-[0-9]{1,}\)"
        .MatchWholeWord = False
        .Format = False
        .Forward = True
        .MatchWildcards = True
        .Wrap = wdFindStop

        Do While .Execute
            If rng.Font.ColorIndex <> wdRed Then
                Set innerRng = rng.Duplicate
                innerRng.MoveStart wdCharacter, 2
                innerRng.MoveEnd wdCharacter, -1

                innerRng.Font.Color = wdColorBlue
                innerRng.HighlightColorIndex = wdYellow
                innerRng.Font.Bold = True
                innerRng.Font.Size = 12   ' ← set font size 12 here
            End If

            If rng.Start >= ActiveDocument.Content.End Then Exit Do
            rng.Collapse wdCollapseFound
        Loop
    End With

End Sub

This ensures only the found inner range is formatted as blue, bold, highlighted, and size 12.

References:

• Applying formatting to text
• Finding and Replacing Text or Formatting
• Sample macro that uses a loop to find text in Word documents
• Programmatically search for and replace text in documents