Microsoft: Increase the Size of a Database

You can increase the size of a database in SQL Server 2014 by using SQL Server Management Studio or Transact-SQL. The database is expanded by either increasing the size of an existing data or log file or by adding a new file to the database.

<< Using SQL Server Management Studio >>
To increase the size of a database:

1. In Object Explorer, connect to an instance of the SQL Server Database Engine, and then expand that instance.
2. Expand Databases, right-click the database to increase, and then click Properties.
3. In Database Properties, select the Files page.
4. To increase the size of an existing file, increase the value in the Initial Size (MB) column for the file. You must increase the size of the database by at least 1 megabyte.
5. To increase the size of the database by adding a new file, click Add and then enter the values for the new file. 
6. Click OK.

<< Using Transact-SQL >>
To increase the size of a database:

1. Connect to the Database Engine.
2. From the Standard bar, click New Query.
3. Copy and paste the following example into the query window and click Execute. This example increases the size of the file test1dat3.

Transact-SQL
USE master;
GO
ALTER DATABASE AdventureWorks2012
MODIFY FILE
    (NAME = test1dat3,
    SIZE = 20MB);
GO

Reference:
Increase the Size of a Database
http://msdn.microsoft.com/en-us/library/ms175890.aspx