This is a GroupHeader and its Width or Height is determined by the available space in the view definition. If you wish to style or customize the GroupHeaders, please be sure to check this help article
on the topic.
In particular, I believe you are referring to the VerticalBottomLevelGroupHeaderStyle.
I hope this will be helpful.
the Telerik team
Sharpen your .NET Ninja skills
! Attend Q1 webinar week and get a chance to win a license! Book your seat now >>