Appropriate tempdb size? DBCC CHECKDB is failing on a particular database because of 

Could not allocate space for object 'dbo.SORT temporary run storage:  143479689576448' in database 'tempdb' because the 'PRIMARY' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.

Tempdb is currently 5GB. The database in question is 56GB. The biggest table in that db is 15GB. 

How big shall I make tempdb?

Thanks
You can use the "WITH ESTIMATEONLY" with your dbcc command. 

DBCC CHECKDB WITH ESTIMATEONLY

This should give you an aprox. space in tempdb to run it. .