The container size I’ve used in this recipe is 4 inches x 4 inches x 3.35 inches.  This fits a 3.5 inch x 3.5 inch x 3 inch cake nicely.  The cake recipe here makes a cake tier about 1 inch high which is perfect for 2 tiers plus frosting.  Rule of thumb is to get a container about 0.5 inches larger in all dimensions than your cake.  This will allow you to add a piece of parchment paper underneath for easy removal plus ample room at the top.