While the Web Illustrator program is open, load the file in question that you are trying to adjust. In the top right hand box there is a drop down box labeled "Tools" open it and select properties.
You just have to go to the image tab. This tab will lets you resize the image and save for Web dialog. You can change Width and Height to change equally, you can also check the Constrain Proportions box. You can resized also by percentage.
