Games being 50GB or even 30GB on average is a load of rubbish.
Fifa is around 12GB as is War Thunder.
Lego is around 4-6GB
BF4 is around 45GB with DLC, Killzone is around 40GB without DLC.
your talking about a 20GB average on a full PS4 game, PSN games like dead nation, resogun are tiny so they bring the average down further but I'm going to ignore them as they aren't "proper" games but if I were to include them the average would be like 10-15GB.
Over the years I could see the average increasing to say 30GB which is a 50% increase on the current average of around 20GB, but even then It's hard to see in games like fifa where all that extra data is going to come from unless they start adding a lot more to them.
a 500GB drive should easily do 10-20 games.
personally I'm waiting for the price of a 360 GB SSD to hit <£75 before buying one, I don't need any more than 200-300GB so would get a 300-400GB drive to be on the safe side.