Vertical sizing of a wxRadioBox

When using wxRadioBox with style= wxRB_SINGLE, the box surrounding the
buttons is too tall. I have tried specifying a size, but this only seems
to affect the width and not the height. Is this a function of the sizers
instead? The code I am using to create the RadioBox is this:

#Create the radio box that displays the input source options. wxRB_SINGLE
makes them appear side by side
        self.source=wxRadioBox(self, -1, majorDimension = 1,
            label='Select input source:',
            choices=['Select command below','Read input from file'],
            size=(280,0), style= wxRB_SINGLE)
        EVT_RADIOBOX(self, -1, self.SetMode)

box = wxBoxSizer(wxVERTICAL)