In the "Add Media" dialog box the following images sizes are available.
Thumbnail (80 × 80)
Medium (300 × 200)
Large (1024 × 682)
Full Size (xxxx × xxxx) (changes depending on full size image uploaded.
I have found the choices full width when set at max is about 500 pixels. Many times we will want to put in a full width column image. To select the large and use the handles is a bit awkward and adds to page load time.
I went off to edit functions.php but stopped when I found many more items that relate to this image than I saw the last time I edited on on a site. Must be the responsive re-size function makes this a bit more complex.
I am wondering why, out of the box choices would not have had the full width size as an option?
Plus how can I add a full width column image selection to the "ad media" dialog box and then into a post without breaking the great responsive features of Choices.
Create a new thread and tell me what problems you are encountering and what you want to know. 
There is no step 3 :) 
