There are serveral ways to find out database size. Apart from using the SSMS GUI, you can using following scripts options.
--this return size info for all databases
--the size is in kilobytes
exec sp_databases
--this return size info for all databases
SELECT
DBName = DB_NAME(database_id)
,FileName = name
,physical_name
,type_desc
,Allocated_Space_GB = CONVERT(DECIMAL(18,2),size * 8.0/1024/1024)
--,max_size
--,growth
--,is_percent_growth
FROM
sys.master_files
WHERE
database_id > 6
AND type IN (0,1)
--this return size info for current database with free space info
SELECT
DBName = DB_NAME()
,FileName = name
,physical_name
,type_desc
,Allocated_Space_GB = CONVERT(DECIMAL(18,2),size * 8.0/1024/1024)
,Free_Space_GB = CONVERT(DECIMAL(18,2),(size - FILEPROPERTY(name,'SpaceUsed')) * 8.0/1024/1024)
--,max_size
--,growth
--,is_percent_growth
FROM
sys.database_files
WHERE
type IN (0,1)
--this return size info for user databases with free space info
EXEC sp_MSforeachdb '
USE [?]
IF DB_ID() > 5 --Exclude system databases
SELECT
DBName = DB_NAME()
,FileName = name
,physical_name
,type
,Allocated_Space_GB = CONVERT(DECIMAL(18,2),size * 8.0/1024/1024)
,Free_Space_GB = CONVERT(DECIMAL(18,2),(size - FILEPROPERTY(name,''SpaceUsed'')) * 8.0/1024/1024)
--,max_size
--,growth
--,is_percent_growth
FROM
sys.database_files
WHERE
type IN (0,1)
'
--find the used data size of all databases
IF OBJECT_ID('TEMPDB..#Result') IS NOT NULL DROP TABLE #Result
CREATE TABLE #Result(DBID INT,DBName sysname,FileName varchar(100),PhysicalName varchar(100),type varchar(10),Allocated_Space_GB decimal(18,2), Free_Space_GB decimal(18,2))
INSERT INTO #Result
EXEC sp_MSforeachdb '
USE [?]
IF DB_ID() > 5 --Exclude system databases
SELECT
DB_ID()
,DBName = DB_NAME()
,FileName = name
,physical_name
,type
,Allocated_Space_GB = CONVERT(DECIMAL(18,2),size * 8.0/1024/1024)
,Free_Space_GB = CONVERT(DECIMAL(18,2),(size - FILEPROPERTY(name,''SpaceUsed'')) * 8.0/1024/1024)
--,max_size
--,growth
--,is_percent_growth
FROM
sys.database_files
WHERE
type IN (0,1)
'
SELECT
DBName
,Data_Size_GB = SUM(Allocated_Space_GB) - SUM(Free_Space_GB)
FROM #Result
WHERE
DBID > 4
and type = 0
GROUP BY
DBName
ORDER BY
Data_Size_GB DESC