The size of the screen replacing the newswall, would depend on what camera shots are being planned. With the width of the last one, you could often see a lot of the top and bottom, but if zoomed in you don't get all the width in etc.
Also if they plan to do more close up shots of the presenter at the screen, then in order to look crisp in resolution, a less wide screen is needed.
Baring in mind the presenters probably won't be walking along it (Kay Burley, Afternoon Live style), it doesn't really need to be the length of a bus.
Will be interesting to see how the touch-screen capabilities work.
BTW, does anyone know when the old newswall was last seen on screen (before it was taken down).