A lot of the time one gets confused with the size of these super robots because of their proportion to different things.  The best thing to do I think is to see how big are people compared to a super robot.  Grendizer is definitely bigger and taller than Great Mazinger.

Duke Freed and Hikaru stand at the bottom.  But you need to realize that you are looking up at Grendizer in the above pictures.  Duke Freed fits into the mouth of Grendizer all the way up there !

Here you see Duke Freed and how he compares with Grendizer.

Grendizer holds Midori after winning a battle.  I think Grendizer's face looks pretty peaceful, yet tough at the same time.

Grendizer holding Koji and Hikaru.

Again we see some proportion here.

We see Shinishi inside a capsule in Grendizer's hand.  I think he's quite safe there.

Again you can see Shinishi and his size compared to Grendizer's hand.