After you perform the appropriate analysis, you'll reach one of two conclusions. Either you have enough site traffic to warrant charging for banner ads, or you don't. Let's suppose for a moment that you do. You need to get with the people who maintain your Web site to discuss exactly how the new banner ads will be incorporated into the site. There are two common approaches.
Some sites place a single banner ad on any given page.
A graphic of about 450 pixels wide by 60 pixels high is common for this sort of use. You don't want to over-do it, but two or three of these on a single page is not uncommon. For this type of advertisement, the average size is around 140 pixels wide by 60 pixels high. You also need to cover the technical aspects of using banner advertisements. For example, you may want to put more than one banner in a rotation in the same physical location on a page.