Here you will find two blank templates to be filled in, they have different aspect ratios, and as the idea of making a storysketch before making a film is to see how the story will be told, how the story will be framed, the format in wich the film will be shown must be same in wich the storysketch is drawn.
The first template has the must common anamorphic proportions, from the outer frame to the inner frame, the proportions are: 3,55:1 (on HDTV scale is 1920X540 pixels). then it comes the wider Cinemascope in 2,66:1 (on HDTV scale is 1920X720 pixels), and then the less wider Cinemascope on 2.35:1 (on HDTV scale is 1920×820):
The second template is in the Four thirds standard format (the format of the oldie SDTV, NTSC-PAL )
Both templates are in letter size.
To know how to use these templates please refer to this article: The Storysketch