Resize embed on mobile only

Site URL: https://www.hoosieraction.org

hi, 

I am having trouble resizing an embed block on mobile only. I'm not sure if the code I need should go within the embed block code snippet or custom CSS. I have tried to enter custom CSS for a few things, but have only been successful in resizing within the code snippet itself. Subsequently, all newsletters w/i our archive have this issue, so, if there is a way to mass-adjust, that'd be great, but I can also go one by one if need be. Please explain like I'm in 3rd grade because that's my level of understanding code 🙂 

https://www.hoosieraction.org/newsletter 

In the iFrame, you have the width set to 900 change this to 100%.

You may need to adjust the block size on desktop as it may appear smaller initially due to the current layout.

thank you!!