An HTML page can be as wide as you need it to be. Since HTML is not a "printed" or "paged" destination (HTML doesn't have page numbers), I'm not sure what you mean by "making the total page width more narrow" -- do you mean when viewed in a browser???
There is an outputwidth or width style attribute that is generally used to alter the width of the ODS output that's written to the HTML file. I'm not sure how it works in conjunction with ODS HTMLPANEL. You might wish to check with Tech Support on this question (whether changing OUTPUTWIDTH in a style template) will impact the overall width of HTMLPANEL output).
If you resize your IMAGES using SAS/GRAPH options or ODS GRAPHICS options, that might also impact the overall width of the HTMLPANEL table.
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).