The size of the images can depend on your Prestashop theme. But you can try something from admin, first.
From your admin page, go to preferences > image tab. You should have a “home” value. You can edit the width/height.
Click save and check how your front page looks.
If the images are having the same size as before, it is possible that they are taking their size values from a custom theme.
You can check the themes folder, look for your current theme > modules > homefeatured folder.
That folder should contain a .tpl file that can be edited. Also you should check the css files.
These instructions are for Prestashop 126.96.36.199.