I noticed lots of articles on how to add custom sizes for images (media).
But how would I be able to add a custom size such as "width:100% and height:auto" this way it will make sure the images are responsive.
Of course, I don't know what articles you've read - however, I'm pretty much sure they are about WordPress' functionality to automatically resize images after upload, as in actual pixel width and height. What you are talking about, is scaling (via CSS).
So in short: these are two different pair of shoes.