ISP Twitter is conceived to perform optimally across as many screen resolutions as possible.
Here is how we came up with the background sizes, in kb and in pixels.
1. File size: official Twitter guidelines recommend a file of less than 2mb
We did our best to achieve that. The lawyer template you attached is about 900kb.
2. Background size: there is no "right" or "wrong" background size, because everyone has a different screen size and resolution.
We decided to settle on the most common resolution so that your designs could show accurately on as many different computers as possible.
The latest StatCounter data show that the most popular resolution is 1366x768 since July 2012
So we optimized for that screen resolution.
We recommend to "center" your design and select a matching background color on your Twitter design settings to make up for people with bigger screens.