[Javascript] Determining the window sizes

[Trev and Liz Smith]

I am relatively new to JavaScript but am able to find my way round the 
DOM model OK.
I am trying to write scientific calculations in JavaScript and plot the 
results.
I have a set of plotting functins OK but I want to size the graphs to 
fit nicely on the page with other text etc.
To do this I need to find the space available to me on the user's screen 
at run time.
I am finding it difficult to get the available area's dimensions in pixels.
Can someone help me by advising me where and when to get the 
window.document.body.clientWidth property?

Regards,
Trevor