The following programs prints the correct size:

import time
import vtk
renWin = vtk.vtkRenderWindow()
renWin.SetSize(30000, 20000)
renWin.Render()
print(renWin.GetSize())

=============================

import vtk
contextView = vtk.vtkContextView()
renWin = contextView.GetRenderWindow()
renWin.SetSize(30000, 20000)
renWin.Render()
print(renWin.GetSize())

=============================

which is (on my machine):
(1920, 1016)