Okay I'll just give you a summary......
Choice of layout containers depends on the layout of your dashboard. I'll advise have a mockup of your entire dashboard on paper. Understand what views you want to have as standalone and ones you want to put into one container. For those you want to put into one container, then decide how you want them arranged. Do you want them to be place side by side? or one on top of the other?
Vertical layout containers allow you to place objects one on top of the other. Horizontal layout containers help you to place objects side by side. Blanks can be used to separate 2 containers or objects within one container. You can nest a vertical container into another vertical container or a horizontal container and vice versa.
You can decide how your graph looks in the containers by setting the "Fit". You can fit to normal, width, height or entire container view. Having scroll bars result when the content of your view is longer than the container. You can adjust the height of your container to eliminate the scroll bar or at least control how much content is shown before the scroll bar. One thing to bear in mind is that you can fix the size of a container. You can fix the height of a vertical container and the width of a horizontal container.
One more tip is that it can be tricky sometimes putting multiple objects into a single container. This is due to height limits imposed by the dashboard size. To overcome this I often start by initially setting my dashboard height very high. Then I can freely play with all the objects. If you go this route, remember to lock the heights of your finished containers before resizing the dashboard height to avoid squashing all the objects.
It's always a struggle wrapping one's head around Tableau's containers the first time but with practice it becomes very easy.
You may find the resources below useful.
Hope this helps,