You should modify width or height property of RadGridResizer
element in order to make bigger grips everywhere inside RadDocking
control. You can do that by taking the resources from the RadControls
installation folder on your machine. Inside that folder you will find the Themes folder which contains all themes resources. Just drill down to the Telerik.Windows.Controls.Docking.xaml file and theme of choice and include those resources in your project.
Hope it helps.
All the best,
the Telerik team