We are using Bootstrap. So all the elements in our theme is responsive and will take space according to your content. The slider width gets defined by the image width; and the height of the slider gets defined according to the image width. Although there is no “ideal” size for the slider image but you can try with 820px*350px.
If you don't want to have the right sidebar and make the slider more wide, you can comment out line 129 in front-page.php (that removes the right sidebar) and change col-md-9 to col-md-12 at line 17 of front-page.php (that makes the slider more wide)