The “end user” way is to use TileEditor and edit any page by clicking on the green dot on the tab up top. Change “Effect Scope” from This page to All Pages (there are two versions - pick the first one). Then select “panel” in the box on the right so you are changing the whole panel, and change the overall tile size width to 1280 and the background size to either auto or 1280. Click Okay. This will set the page size of all pages and store the setting in your customtiles.css file.
The fast “power user” way is to edit tileeditor.css and change the body style in lines 14 through 18. Change width from 1190 to 1280
If you choose the power user method, you can still use tile editor method and override this. You could also change the “greeting” and “greetingopts” styles so your auth page makes use of the entire width of your tablet, but you probably don’t care about that.