I experienced the same issue with Miro App on iPad these 2 weeks as many of your in the community.

iOS 18.1
iPad Air (4th Generation)

In my Maths lesson, I have uploaded a PDF with around 40 pages. It was really smooth before. 

I tried to delete 30 pages and all the written contents. The app still told me the board is too large.

I have tired to delete the app and re-install it. It has still happened. It really drives me crazy.

Can you please help with my case?

Thanks.