For NVARCHAR2 and VARCHAR2 maximum size is 4000 bytes, or 32767 bytes if the MAX_STRING_SIZE initialization parameter is set to EXTENDED. This is useful you have to allocate more data to a variable.

You can run the below command to view the parameter.

show parameter MAX_STRING_SIZE;

References

Oracle Doc, Max string size

In Oracle PL/SQL, you can use the following commands to view table details:

1. DESCRIBE or DESC

   DESC table_name;
   

   Displays columns, data types, and constraints.

2. USER_TAB_COLUMNS

   SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
   FROM USER_TAB_COLUMNS
   WHERE TABLE_NAME = 'TABLE_NAME';
   

   Shows details about columns for tables owned by the user.

3. ALL_TAB_COLUMNS

   SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
   FROM ALL_TAB_COLUMNS
   WHERE TABLE_NAME = 'TABLE_NAME';
   

   Provides column information for tables accessible to the user.

4. DBMS_METADATA.GET_DDL

   SELECT DBMS_METADATA.GET_DDL('TABLE', 'TABLE_NAME') FROM DUAL;
   

   Displays the CREATE TABLE statement for table structure details.

To view constraints on the table, you can check here.