You can't make a variable to have these styles. But since you probably want to display that text then just use ummm... let's say RichTextBox control.
Code:
Private Sub Form_Load()
    With RichTextBox1
        .Font.Bold = True
        .Font.Size = 24
        .Text = "I want to Make this sentance Bold and bigger"
    End With
End Sub
I guess that it's not what you are looking for, but I thought it would help