In the current version of the cart v3.86 this is fixed. You have the option of upgrading to the latest version to fix the issue or if you dont want to upgrade you can open the productdetail.ascx located in \desktopmodules\smith.buynow and search for "TabContainer" or go to line 195 and set the width of the description labels as highlighted below:
<asp:Label ID="lblDescription" runat="server" CssClass="SmithProdText" Width="400px" />