To calculate the height of the giant, we must make the assumption that we want the giant to be tall enough to pick upJack in one hand. We can not measure the height of the giant, but we can see that his hand covers Jack's body from under the arm to the top of the leg just past the hip.
Now, we need to measure Jack. First his height.
Then across his kuckles.
And, finally, from under his arm to the top of his leg, just past the hip. This is where the giant's hand would cover Jack's body.

 

 

Now, we are ready to set up the proportion to find the height of the giant.