PostgreSQL equivalent to SQL Server NVARCHAR (MAX)

If you want to created an "unbounded" varchar column just use varchar without a length restriction.

From the manual:

If character varying is used without length specifier, the type accepts strings of any size

So you can use:

create table foo
( 
  unlimited  varchar
);

Another alternative is to use text:

create table foo
( 
  unlimited text
);

More details about character data types are in the manual:
http://www.postgresql.org/docs/current/static/datatype-character.html

In PostgreSQL for char(n) and varchar(n) it is just limited by n argument (not sure about nature of such limitation, it seems that it is 20 Mb or 10M of 2-bytes characters or something):

postgres=# select 'a'::varchar(999999999);
ERROR:  length for type varchar cannot exceed 10485760

So if you really need the equivalent for MS SQL Server nvarchar(max) - it could be varchar(10485760)

However to use really unlimited character values there are two ways: varchar or character varying without argument or just text type. Read more in the documentation and note about Tip provided in it:

There is no performance difference among these three types, apart from increased storage space when using the blank-padded type, and a few extra CPU cycles to check the length when storing into a length-constrained column. While character(n) has performance advantages in some other database systems, there is no such advantage in PostgreSQL; in fact character(n) is usually the slowest of the three because of its additional storage costs. In most situations text or character varying should be used instead.

text, varchar can be used for this requirement. i used varchar

Character Types Description

character varying(n), varchar(n) --- variable-length with limit

character(n), char(n) -- fixed-length, blank padded

text, varchar -- variable unlimited length

More Details: https://www.postgresqltutorial.com/postgresql-char-varchar-text/