The value field of a DNS record can not exceed 255 characters
This functionality can be considered as a feature request and it is already registered as one with ID #1972412. It will be implemented in future Plesk versions.
You can vote for this feature on Plesk User Voice Plesk User Voice portal.
DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION page 9 paragraph 2.3.4. Size limits
Various objects and parameters in the DNS have size limits. They are listed below. Some could be easily changed, others are more fundamental. labels 63 octets or less names 255 octets or less <------ TTL positive values of a signed 32 bit number. UDP messages 512 octets or less