You can use these images sizes as a guide – but you need to look on the site to see how large or small you want them to display.
Also remember if changing to Big Shot from another theme you can use the Regenerate Thumbnails plugin.
Home page slideshow – 1600px wide x 800px (check after you’ve uploaded the first one – you may want to go larger!)
Blog – 500px wide
Page header – approx 1200px wide x 400px
Portfolio – approx 1200 px wide x 800px