Banner sizes explained
There are 3 banner sizes for you to choose form. It is important to know these are not fixed sizes. Each time you select a banner you still have the ability to change heights associated with the banner.
These sizes as you seem them relate directly to your Style settings for each banner. Your style settings control the styles in each section. For example, a 'Large' banner may be your main hero banner on the home page, and require much larger fonts and different colours to other areas of the site. Being able to control your styles in this way allows for better consistency throughout the site.
Banners will always be 100% width of the section they are placed into. If you wish a wider or narrower banner, change the section the banner sits within to control the different width.