Hi Jonny
Changing the default units will not have an effect on the default shape size. Unfortunately there is not a way to edit the default shape size in Lucidchart - the shapes dragged on to the canvas will always be dragged onto the canvas with the same dimensions.
We’re very interested in your feedback and committed to continually improving our products. If you’re willing to share, we’d love to hear more details about your use case or what you’d like to see in this experience within this thread. I’ve also converted this post to an idea so that it’s visible to others within the Product Feedback section of the community - from here, they can upvote it and add details of their own. For more information on how Lucid manages feedback via this community, take a look at this post.
As a workaround, you can create custom shapes following the instructions in this article. Using the custom shapes option you can resize a shape to the desired dimensions and save the newly sized shape as a custom shape allowing you to drag that shape onto the canvas with the correct dimensions each time it is needed. Hope this helps! Let me know if you have any other questions.