Many users have requested this feature, and the ArtRage team is aware of the demand. While there's no official confirmation on its implementation timeline, they're actively working on improvements for the iPad version.

Here are some helpful alternatives until then:

Resize after creation: You can resize the canvas after creating your document. While not ideal for initial composition, it allows adjusting DPI for printing or exporting.
Pre-sized templates: Create templates on your laptop with desired DPI and import them into the iPad app. This saves time and provides consistent sizes for specific projects.
Third-party apps: Consider apps like Procreate or Affinity Designer, which offer direct DPI setting on iPad. They might not have all the ArtRage features, but they provide a DPI option.
Remember, the AirDrop method remains a reliable workaround for now. Hopefully, the ArtRage team will introduce native DPI setting in future updates.

Here are some resources you might find helpful: