HTML is really a "virtual" page. If you want to make the page less wide, you will need need to shrink the contents of the page. For HTMLPANEL, you will want to use GOPTIONS XPIXEL and YPIXELS to change the dimensions of the graphs you generate, causing the page to be less wide. Also, you can use the "panelcolumns" option in the tagset to set the number of columns in a panel layout (the default is 2 columns).
Thanks!
Dan