This is a little difficult to answer, not being able to see what it is you're working with and being given no details.
Try activating your chart, then drag the herringbone handles until they stretch the width of your slide -- or even hang off the edges a bit.
Then size your plot area to taste.
Right-click your data series and use the gap options to help with resizing a bit. Also, on the font tab, deselect AutoSize so that your font size doesn't grow or shrink when you resize the chart.
If you have extra blank areas to the right -- where data could be plotted, but there's no data to plot -- then use View/Datasheet, select the empty columns, and Edit/Delete to get rid of the empty space.
