The following script will help you in finding the file size of current tempdb and the size when SQL server last restarted.
SELECT
alt.filename
,alt.name
,alt.size * 8.0 / 1024.0 AS originalsize_MB
,files.size * 8.0 / 1024.0 AS currentsize_MB
FROM
master.dbo.sysaltfiles alt INNER JOIN tempdb.dbo.sysfiles files ON
alt.fileid = files.fileid
WHERE
dbid = db_id('tempdb')
AND alt.size <> files.size