Container

Hi I can’t change the size of a container when they are place in a row, as shown in the new tuturial. I am on a Mac!

Size is the sum of all child containers. You need to add at least one container. 

I think I don't understand your anwser, I have created three container under the roote container and I have check ( keep child container in a row). When I have done that I can't change the lenght of the continer at all. I try to follow the tutorial very close.

Check that box for the parent container (root in your case).

You probbaly have selected a child and ticked the box there. Every container can be the parent of multiple children kept in a row.