Today I needed to create a custom property in SharePoint which would store some long strings. So naturally, a question came up: How long can a string be before SharePoint refuses to recognize it? It appears that the limit is set at 2,000 characters. When I tried setting the string field length to 2,000 characters and attempted to save the new property, SP would reset it back to the default value of 25. However, it does accept 1,999 characters or less (see screenshot below).
The SharePoint Nomad 
That’s true if the value is not multivalued. I cannot create a multivalued string property bigger than 400 characters…