To avoid the "crop the PDF" step mentioned in several answers, you can put the chart on its own Worksheet with an appropriate paper size and zero margins. E.g. in Windows Excel 2007:
- Select chart, from pop-up menu choose Move Chart... | New Sheet
- On the new Chart Sheet, Page Layout | Margins | Custom Margins, and zero all
- Resize the chart to the desired size by dragging its bottom-right corner
- See the resulting size from Chart Tools | Format | Size
- Select a close size from Page Layout | Size, and adjust as necessary
If there is no size close enough, or you want to be exact, you'll need to create a custom page size. Unlike Word, Excel doesn't let you do that itself, but rather uses the size from the printer driver.
- Page Layout | Size | More Paper Sizes | Print..., select a printer whose driver lets you create custom page sizes - most normal PostScript or PDF drivers do
- Options... | Advanced... | Paper Size | PostScript Custom Page Size
- Set the size, taking care to get the units right and swap width and height (Excel prints as landscape by default)
It may be best not to have an outline around the chart in the PDF itself: you can choose Format Chart Area | Border Color | No Line.