I knew I forgot something... This code should work for OpenGL or Sreed... At least I hope that it does ;-)

import poser
if poser.Version()=='8.0':
        import wx.aui
def GetOpenGLCanvas():
        l=[]
        for child in poser.WxAuiManager().GetManagedWindow().Children:
                GetOpenGLCanvasRec(child, l)
        return l
def     GetOpenGLCanvasRec(win, l):
        for child in win.Children:
                if child.Name in ['PoserGLCanvas', 'PoserPBCanvas']:
                        l.append(child)
                else:
                        GetOpenGLCanvasRec(child, l)
def GetOutputRes():
        s=poser.Scene()
        if poser.Version()=='8.0' and s.RenderDimAutoscale()==2:
                for win in GetOpenGLCanvas():
                        if win.IsShown(): return win.GetSize()  
        return s.OutputRes()

print GetOutputRes()