I don't believe you change the width/height of a checkbox, since it can't be done in HTML and the checkbox used by the TreeView is a standard checkbox control. The only way I see you having the ability of changing the RadTreeView's checkbox size would be to enable TriStateCheckboxes. When these property is set to true, it uses images for the checkboxes. It uses the following class names for the checkboxes when in that mode: rtChecked, rtUnchecked, rtIndeterminate.