In the Monero URI, is the tx_description stored on the blockchain? If so, is it encrypted? What is the maximum length of this field?
A monero: URI is not a transaction, it is a descriptor for how to create a transaction with some particular parameters.
The description is not stored in the blockchain. Of course, some software might decide to put this in a transaction, but that's on it.
It is not encrypted.
There is no defined maximum length, but some parsers might have limits.