We advise you not to exceed the dimensions 550 x 200 px for your banners.
More generally, the first reflex to have is to think of your email signature as a whole. So if your signature is 500 px wide, opt for a campaign of the same width.
When you create your templates, consider running tests to see how much space variable data occupies. A function or a first name / last name can, for example, occupy more or less space.