I think this code will do what you want...

Sub FindSpecial()
Dim oFind As Object
Dim rRange As Range
Dim sFind As String
Dim c As Integer, l As Integer

'Set the column with your data
'If your data contains more than 20,000 records then adjust the range
'Be sure to select your Sheet name as well
Set rRange = Sheet2.Range("B1:B" & Sheet2.Range("B20000").End(xlUp).Row)

With rRange
    'Find the cell containing the (
    Set oFind = .Find("(", LookAt:=xlPart)
    If oFind Is Nothing Then GoTo CleanUp:
    sFind = oFind.Address 'Prevent endless loop
    Do
        'Find the start and end of your data to resize
        c = InStr(1, oFind.Value, "(") + 1
        l = InStr(c, oFind.Value, ")") - c
        'Select those characters and resize
        oFind.Characters(c, l).Font.Size = oFind.Font.Size - 1
        Set oFind = .FindNext(oFind)
    Loop Until oFind.Address = sFind
End With

CleanUp:
Set rRange = Nothing
Set oFind = Nothing
End Sub