It looks to be 423 characters based on the last time I pulled those codes for my post
;WITH ERRORS ([Hexadecimal code],[Decimal Code],[Symbolic Name],[Description]) AS
(
SELECT '0x8002F347',-2147290297,'DTS_E_STOREDPROCSTASK_OVERWRITINGSPATDESTINATION','Overwriting Stored Procedure "__" at destination.'
)
SELECT
MAX(LEN(E.DESCRIPTION)) AS DescriptionLength
FROM
ERRORS E;
I'd just round up to 450/500/512 for your purposes