Vpython uses the webbrowser package to launch a browser.
In this documentation it talks about setting up an environment variable called BROWSER containing the path to your browser and %s. You could try adding --window-size=800,600 and --window-position=200,300 flags to the command line for the position and size of the browser window you want.
You can launch a browser with a command like
"C:\Program Files\Google\Chrome\Application\chrome.exe" --window-position=800,600 --window-position=200,300
and then try add a %s to this command in and put in in the BROWSER environment variable on your computer. Then run vpython program and see if it opens the browser with the size and location you specified in BROWSER environment variable.