Atticus currently does not support custom trim sizes. This is something we hope to accomplish in the future, but right now we don't have an ETA on this update.
If you're interested in following along with our progress, you can see what we're currently working on and what's coming up next on our RoadMap: https://www.notion.so/Atticus-Roadmap-58d75fe66e0f40e0a7b39192c2cff6c3