Hi,
1. you can use small triangle in the editable window corner (lower-right corner) to change size of window.
2. If you would like to have higher window by default, you can change file
\CMSModules\Ecommerce\Controls\UI\ProductEdit.ascx. There is a control
<cms:CMSHtmlEditor ID="htmlTemplateBody" runat="server" Width="450px" Height="200px" />
so you may change Height pararameter.
Best regards,
Ivana Tomanickova