@guitarthrower is right, you can't grab and resize the chart area - the chart will automatically size itself to the sheet. You can change the size of the sheet though - in the Page Layout tab, Page Setup section, use the Size dropdown. There are a limited number of options here, which are populated from the sizes provided by your printer driver. (If you go to Print, select a different printer, and go back to the sheet, you'll see that the selection of paper sizes will change.)
This is pretty frustrating when you've got a graph that will never be printed, and you just want to scale it up to screen size with no 'dead' space. I found that the Microsoft XPS Document Writer pseudo-printer had a large selection of sizes. And apparently some printer drivers allow you to enter a 'custom' size, but I haven't come across one of those.
Zooming in doesn't help you? – Hauke P. – 2013-11-05T18:46:55.750