What is the maximum data Power BI handles concerning Direct Query and Import Mode?

Hi. All this will depend on the license. You can check in this link a table with max model size memory and max storage limit (single table processing)

https://powerbi.microsoft.com/en-us/pricing/

All that is for import mode. The limit for direct query will be slowliness. If you don't build something really well done and best practices, it will be slow, really slow, but no memory limit because you don't store data.

I hope that helps,