CSS Flexbox Align-items lesson

So in the lesson, how is the height of the boxes calculated?

inside the #boxcontainer (height 500px).

But when you use align-item property, the height of red box and blue box becomes smaller, or perhaps their actual size?

their actual size, correct. look for (width 200px) in the #box-1 and #box-2.

I am asking about height.