Click on the middle button top right of the screen, between the - and x; then put the mouse on the edge of the window and drag it smaller, both side and bottom.
I don't know how to make it EXACTLY 400 x 400; I just judge it by eye comparing it as a proportion of my 1024 x 768 screen resolution.
I would be interested to know if there is a way of measuring the new window accurately.
Put this in the <head>:-
// This will resize the window when it is opened or
// refresh/reload is clicked to a width and height of 400 x 400
// width is placed first, height is placed second
(Just tried it with one of my pages).