SQL Server TempDB size issue
You may want to consider using the query that will show you free space in the log and free space in the data file. If you add them up you'll get the similar value as the SSMS is showing.

This is an excellent query which does just that:

DECLARE @log_used DECIMAL(19,7);
CREATE TABLE #x(n SYSNAME, s DECIMAL(19,7), u DECIMAL(19,7), b BIT);
INSERT #x EXEC('DBCC SQLPERF(LogSpace);');
SELECT @log_used = u FROM #x WHERE n = DB_NAME();
DROP TABLE #x;

DECLARE @data_used DECIMAL(19,7);
SELECT @data_used = SUM(a.total_pages)*8/1024.0
FROM sys.partitions AS p 
INNER JOIN sys.allocation_units AS a 
ON p.[partition_id] = a.container_id;

;WITH x(t,s) AS
( 
  SELECT [type] = CASE 
    WHEN [type] IN (0,2,4) THEN 'data' ELSE 'log' END, 
    size*8/1024.0 FROM sys.database_files AS f
)
SELECT 
  file_type = t, 
  size = s,
  available = s-CASE t WHEN 'data' THEN @data_used ELSE @log_used END 
FROM x;


Query Source and answer to similar question

By the way, it is usually good idea to split data file in your tempDB into number of equally sized chunks (number depends on the number of cores). It tends to help performance a lot. Just don't split the log file (it will do more harm than good).