For the 3-D birthday cake I did recently I used a board to both support the arms on the figure and to help gauge where I needed to taper the cake.
I did the bottom section, and put the board for the top section on top of it to use as a guide. The edge of the board showed me where I needed to taper the bottom block to. I carved the bottom to match the width of the top board.
|Top section after carving|
I covered each section separately in fondant then stacked them. The head was Rice Krispie treats since it had to sit on a skinny neck, and I could make it in one solid piece that way.