Does increasing the tempdb file size require a restart
Increasing the size does not require a restart.

A small example on my dev environment

USE [master]
GO
ALTER DATABASE [tempdb] MODIFY FILE ( NAME = N'temp2', SIZE = 4096MB )
GO
GO
ALTER DATABASE [tempdb] MODIFY FILE ( NAME = N'tempdev', SIZE = 4096MB )
GO


From:



To:





If you are decreasing the size with MODIFY FILE it would require a restart.
If you want to decrease the size immediately, you would have to start shrinking (not ideal).