Panel seems to ignore the size parameter. Sample:
4.1.1 msw (phoenix) wxWidgets 3.1.5
Win 10
import wx
app = wx.App()
frame = wx.Frame(None, title="Title")
panel = wx.Panel(frame, size=(500,500))
frame.Show()
app.MainLoop()
The resulting window is the same size regardless of the values given to size.