I always partition my drive for 10GB on / and let the rest be /home. Never ran into any issues with space on /, even after installing different DEs at once.
If you will be installing games that have native GNU/Linux support, they will
go onto the / partition by default. If you intend on doing that, there might
be a reason to keep it relatively big, yet I'd still resize it if I were you.
30GB ought to be more than enough EVEN if you put a lot of games on there.
You can barely fill 10GB on / with normal software. And you would need to
install most of everything in the repository to do that...
