By default, when RadCheckedDropDownList has fewer items than the specified maximum items, the popup is sized considering the items count. However, when you set the ShowCheckAllItems property to true, this add one item more which is not calculated in the popup's size. Please refer to the attached sample project.

Workaround: set the DropDownHeight property:    this.radCheckedDropDownList1.DropDownHeight = this.radCheckedDropDownList1.ListElement.ItemHeight * (this.radCheckedDropDownList1.Items.Count + 1) + 1;