Disk Size Considerations for a SQL Server 2008 R2 database
You will need exactly the same disk space for your mirrored (secondary) database as for the primary database. They are the same and the secondary database will have the same growth rate as your primary database and should be provisioned the same. 

SP_SPACEUSED will show you how much space your primary database is now using (you should make it larger and not rely on autogrowth), and you can grab growth information from the default trace to give you some limited information about how often the database has grown, you can then infer the growth rate from the autogrowth parameters. 

DECLARE @path NVARCHAR(260);

SELECT 
   @path = REVERSE(SUBSTRING(REVERSE([path]), 
   CHARINDEX('\', REVERSE([path])), 260)) + N'log.trc'
FROM    sys.traces
WHERE   is_default = 1;

SELECT 
   DatabaseName,
   [FileName],
   SPID,
   Duration,
   StartTime,
   EndTime,
   FileType = CASE EventClass 
       WHEN 92 THEN 'Data'
       WHEN 93 THEN 'Log'
   END
FROM sys.fn_trace_gettable(@path, DEFAULT)
WHERE
   EventClass IN (92,93)
ORDER BY
   StartTime DESC;