You can create a custom sized ad unit in the UI, and target a creative that matches the same size.
To create a custom order/line item to serve custom ads to your ad unit, please follow the documentation below.
You can also set up your own direct serve creative by creating an Order and Line Item and adding the creative of the ad format you are trying to test.
To do this, follow the documentation below to set up a Order and Line item and also add a creative to the line item.
https://dev.twitter.com/mopub/ui/setup
https://dev.twitter.com/mopub/ui/settings
https://dev.twitter.com/mopub/ui/adding-creative
