use the ascx filein the div tag or panel on the page to set the width and height.
like this :
<asp:Panel ID="panelgrid" runat="server" ScrollBars="Auto" style="overflow: auto; padding: 5px; width: 325px; height: 450px; ">
<uc1:CategoryTree ID="ucCategoryTree" runat="server" /> </asp:Panel>
Hope this will help you.
And don't forget to mark as answer if it helps. :)