When ever you set a string value in fluent NHibernate it alwasy sets the DB vales to Nvarchar(255), I need to store quite a lot of long string which are based on user inputs
Probably you are using "NHibernate validator" as well. If yes, Fluent NHibernate will consider all of the NHibernate validator related data annotations automatically, including string length, not null, etc.