Query related to length in CAST and Output clause

Hi Team,

Could you please reply to below queries -

1. Using CAST, I need to convert BigInt and Datetime2 values to Varchar. How could I be sure on the length that I need to use (in CAST) to convert these values to Varchar.
2. I am using Update statement with Output clause, in Output clause values, I am unable to use a user defined function as it is erroring out, could you let me know how to use a user defined function in Output clause (with Update).

Please let me know if more clarification is needed in any of above points.

Thanks,

Salil