Not only can the size of TempDB files be unpredictable (unless the workload is completely predictable or a size limit is placed), but it is full of old Tupperware. That is, if TempDB is destroyed, your SQL Server will create a new one as soon as the service restarts. The whole migration is summarized in these 4 steps:
1. Create a new location for TempDB
2. Use TSQL to change the TempDB file location(s)
3. Restart the SQL Server service during a maintenance window
4. Verify and clean up
This is one of the easier things to move, but it does require server downtime.