The theme is set here to just show five posts on the blog pages. You can set it to any number that you like up to ten. Use the featured image setting for each post to set your slider and featured post images. If you have not previously used featured images, try the Quick Featured Images Plugin to automatically set them. For this theme, featured images that are at least 1240 wide are desired for posts that show in the slider.
If you are having issues with featured images showing in differing sizes, which isn’t often a problem with the them, but can arise when you start using related posts plugins, or when you migrate from a theme that used different sizes for featured images and products thumbnails, grab the Regenerate Thumbnails Plugin to re-size everything.