When a user database is created, if size is not supplied for the primary file, the Database Engine uses the size of the primary file in the model database (default size is 8MB). When a secondary data file or log file is specified, but size is not specified for the file, the Database Engine makes the file 8 MB.
The size specified for the primary file must be at least as large as the primary file of the model database.
SQL Server 2016:
The default Database size is 8MB.
The default autogrowth setting is 64MB at a time with unlimited growth