The last factor that can effect your choice of casket is the design concept you want to do and how that fits on a casket.
The first thing to do if the design concept/story or technique is the most important factor to you is to make a list of everything you want on your casket. Then look at the list and divide it into major items and minor items. The easy way to think about that is to separate the ones that are to be in the background like trees, flowers, cats, castles, etc. and put those in the minor category. The figures like the characters, family members, etc. are in the foreground and they are the major items. Typically, only one major item will fit on a panel and the minor items will be used as the background fillers. You can have two to three on a large panel like the top of a Flat Casket or the back of a piece if you have some that relate in a scene of a story.
Now count up how many of the panels you have. If I was doing my family I would have four people and thus four panels. Then look at the guide in the diagram. Four panels would fit well on a Flat or Short Flat casket if you used something else for the top (like a flower jar).
If you had six different major characters (a family of six, lets say) it would be easier to fit that on a Double Casket with the two doors, two sides, a back and a top which equals six panels.
Hopefully this helps show how your design can lead to a smaller number of casket type options that you can pick from, and then you can use other factors to chose the one you want.
Tomorrow, I will start to talk about other factors that keep us from starting - how perfect is the enemy of the good.