The most common form of web advertising, can have standardized dimensions (Google offers
for its Ads), but also non-standard,
for instance: on your site the banner can have arbitrary dimensions if it fits into the entire design without spoiling the space.
It always starts with the user
he needs to immediately understand what you are offering, under what conditions, without ambiguous or confusing guidelines.
The banner should be readily and attractive at any time, text formatted and the call in action must be clear.
Only such can contribute to a greater site visit and ultimately mutual satisfaction of you as a seller of a product or service provider and your user on the other side.
More than 10 years of experience
in making web banners of all shapes (static or animated) could be just what you are looking for.
and talk about the banner you need.