I have my first real cake order for next week. It's for about 40 people. I've posted a pic of the cake she wants. I need to know if I make the cake 10 x 10 x 3 for the bottom and half of the Wilton Ball Pan for the top, will be enough for 40 people. I do want the pieces to be big enough.

That should work.  A 10x10x4 serves 44, so a 10x10x3 should serve about 32-34.  The dome from the Wondermold serves about 10-12.  If you use the ball pan, bake an additional 6" round to go under it to add the extra servings.

Thank you BeckyRink.