I think it's hard to guess the final size of your TSM database, but it's
easy to expand it.
So I usually start with 2-4 GB and watch it / have a script watch it, and
add volumes as needed.

There is a feature that will grow the database for you, and might work OK on
MVS.
It's the DEFINE SPACETRIGGER command.

Unfortunately for those of us without mainframe-class DASD, DEFINE
SPACETRIGGER doesn't seem to handle mirroring very well.  If you have
database mirror volumes defined, it will create new mirrors for any new DB
volumes it adds - that's good.  But it puts them in the same directory,
i.e., on the same physical disk.  That's bad.  So I do it by hand.