You might be running into the issue of your SnapWidget looking perfect in your editor, but not in your preview or live site.
To change the size of the Instagram widget to have it look like the sizing in the editor, you will need to click into the embed code for the Instagram under the embed code tab on the right side.
(for larger image: Right Click>Open Image in New Tab)
For the height and width, change that to 100% and then click SAVE.
Then publish or preview! Your Instagram widget should be sized just like it is in the preview.