Azure durable entities are stateful components offered by Microsoft Azure functions. They can hold a state, which requires to be JSON serializable, and will be stored in reliable storage.
My questions are:
JayakrishnaGunnam-MTs pricing answer is correct for normal functions but azure durable entities/functions are also billed as per normal storage account costs for storage and transactions to table and queues. https://learn.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-billing
Of what I could find about the maximum size for a single entity the only answer I was able to find was the size of a blob as a limit. I was not able to find another limit in the DurableEntityContext implementation but might be missing something.
UPDATE: After some testing, it does seem that durable entities are for < 64 kb stored inside the table storage. After that, they will be moved to a blob. My simple entity with just a large string was only able to be stored for about 50-100MB but at 100 it started to have some issues. Sometimes I received an OutOfMemoryException. (Consumption plan)