Algo como isso deve funcionar para você:
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Your combo box should be built and populated when the form loads, not when the combo box is changed
ComboBox1.Items.Add("6")
ComboBox1.Items.Add("10")
ComboBox1.Items.Add("12")
ComboBox1.SelectedIndex = 0 'This Auto-Selects the first entry to prevent you having to handle "null"'s
End Sub
Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
Dim tryParseResult As Boolean 'holds the True/False for whether conversion to a number worked
Dim parsedNumber As Integer 'Holds the number after conversion
tryParseResult = Int32.TryParse(ComboBox1.SelectedItem, parsedNumber) 'try converting to number - "parsedNumber" will contain the number on success - "tryParseResult" will contain a true/false for success
'If tryPArseResult = False - conversion failed - tell the user and stop.
If tryParseResult = False Then
MessageBox.Show("Conversion of " & ComboBox1.SelectedItem & " To Number Failed") 'Tell the user it failed
Exit Sub 'Stop processing
End If
'Set TextBox1's Font to the same font with a new size
TextBox1.Font = New Font(TextBox1.Font.Name, parsedNumber)
End Sub
End Class
Se você quiser fazer isso com controles multipler - você precisa olhar para o loop através de controles em um formulário usando OfType
se você quer apenas trabalhar em certos controles