This field can have no more than 255 characters.
Well you should have seen this message if you have a custom multiline column added to the SharePoint document library and try to add a text that is more than 255 Characters. And there is no way around to solve this issue in the SharePoint UI/ at least I didn’t find one so far.
Today I was trying to create a custom list template, and found an attribute called UnlimitedLengthInDocumentLibrary. This is the attribute that will solve your issue. And in case if you don’t want to create a custom List Template to solve this issue , there is a easier way around this.
Hope this helps!!!!