Hi Joran,
Sure I'll be so kind
The width of the grey background shade area is actually determined by the corresponding picture which is to be found in your websites root directory:
/templates/gk_postnote/images/style1 "or" style2/column.png
Standard size is 220x100 ... you can simply alter the width of this picture and upload it again. In the front-end it will show the change then.
Again ... it is fairly simple but I was thinking in the wrong direction at first as I thought that the width is determined in the corresponding K2.css. But it isn't
Hope I could help you out on this, Joran
Greetz