I haven’t heard back from you, so here are some generic instructions on how to add CSS to your site and what a potential solution will look like.
Log into your site and go to appearance -> customize. From here select Additional CSS from the left menu and you will see a new textbox where you can add some custom CSS to your site. Remember to clear your site cache if you are using a page caching plugin!
Since I cannot see your site, there is no way for me to know what is causing your images to be too small. But here is some CSS you can add to your site that might help.
.hdq_quiz {width: 100% !important}
.hdq_featured_image {width 100% !important;}
The first line increases the width of the entire quiz to fit whatever available space is on your page. The second line attempts to force your featured images to fill 100% the space of the section as well incase you are either uploading super small images or your theme is forcing a specific maximum size. Feel free to play around with those 100% values to see what fits best for you. If it still is not right for you, then you will have to show me a link to an active quiz in order to help further.