There are only 3 permanently set sizes, S, M, L. L is more like XS, not really large. This is not usable for Confluence, just to embed a preview, where you can't see anything.
If you are referring to the miro confluence app, I would contact their support team directly instead: https://marketplace.atlassian.com/apps/1217530/miro-for-confluence?tab=support&hosting=cloud
Hi Atlassian Community, My name is DJ Chung, and I’m a Product Manager on the Confluence Cloud team. Today, I’m excited to share a new and improved version of Home. The new Home helps you ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events