Many people ask me, they need to use field holds more than the 255 characters. They used MultiLine text. It worked fine with custom list but failed (generate error can't hold than 255 characters) with content type.
Many bloggers suggest to create custom field type, I tried this suggestion but gave me same result.
I found simplest solution Create Site Column of type MultiLine text, and set property UnlimitedLengthInDocumentLibrary to TRUE.
You can use tool Imtech Fields Explorer v220.127.116.11 to set this property.
download Imtech Fields Explorer v18.104.22.168