Allocation in files belonging to tempdb is done using proportional fill. It means that when the files have the same size the allocation will be distributed across all files. It is very important to have all files set to the same initial size and growth because if there are difference in the size, there will be difference in the load pressure that each file will have. As for the size that you should use – this is something that no one here can tell you what to use. It depends on how much use you have for tempdb and we simply don't know that.

Adi