PAN-OS 7.0 and below:
The limit for a pre-shared key is 64 characters.
owner: panagent
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000CleICAS&refURL=http%3A%2F%2Fknowledgebase.paloaltonetworks.com%2FKCSArticleDetail