In short, you can use any ad size you'd like
With a Sulvo account, you also have the ability to determine a custom height and width of your ads using Fluid units that automatically adjust to the available space, Sticky units, or AMP units.
Sticky ad units
Sticky ad units are available for desktop (Sticky Display) and mobile (Sticky Mobile). Please visit the Inventory page for sticky ad unit sizes.
Fluid ad units
These units have the capability to automatically draw buyers from a larger pool than normal non-Sulvo banner units. It also has the flexibility to fill its available space on any device without having to manually set up or separate units per viewport or device. The maximum ad size is always controlled by your site and you can also control device visibility during the ad code creation step.
AMP
If you have created AMP pages, you can serve sticky or display ads on them. The AMP (Accelerated Mobile Pages) Project is an open-source initiative that aims to make the web better for all. The project enables the creation of websites and ads that are consistently fast, beautiful, and high-performing across devices and distribution patterns.