Don't believe there's any native way of doing this in an Access Form, and believe that handling it thru code would be prohibitively cumbersome/complex.
In this type of situation I always use the Control's DoubleClick event to 'zoom' its size up:
Code:
Private Sub MemoTextBox_DblClick(Cancel As Integer)
DoCmd.RunCommand acCmdZoomBox
End Sub
Simply replace MemoTextBox with the actual name of your TextBox.