The banner ads refer to the advertisement that appears on a website and directs the visitors to another website. These ads appear in box and can have attractive visuals and graphics that can attract people to it. They are generally of different sizes as per the placement of the ad and you can use the one that is apt for you.
If you need to use a leader board banner ad which is the largest size available then the standard banner ad size is 728X90. It can be placed on the top of the website page or at the bottom. Then there full horizontal banner and their adsize are 468X60. If you find them to be expensive then you can choose half banners which have standard adsize of 234X60. Apart from the horizontal banners, advertisers can use vertical banner ads also. The standard size of vertical banners is 120X240.
These are the general sizes and are not restricted. The advertisers have the option buy bigger or smaller advertisement as per their requirement. The banner ads are contextual ads and are easily noticed by everyone. They are also efficient in diverting quality traffic to any website and are therefore highly effective.