Create a wx.Font instance with the face, size, etc.; create a wx.DC; then call dc.GetTextExtent("a text string") on that to get the width and height needed to display that string. Set your row height and column width in the grid accordingly.
Something like:
font = wx.Font(...)
dc = wx.DC()
dc.SetFont(font)
w,h = dc.GetTextExtent("test string")