For best results use high resolution assets. Spaceback creates versions optimized for high-density displays serving @2X assets when possible. Multiply all pixel counts for the end resolution. For example, @2X output, a standard 300x250 banner with a caption should have a media asset of at least 596x280px.

‍

‍

‍

‍

‍

‍

‍

‍

‍

‍

‍

‍

‍

‍

The above creative specs are default settings.
Twitter posts have advanced creative customization options to easily adjust maximum line length. Adjusting the line-length also adjusts the media crop dimensions.