card box

i am looking to do a DIY card/cake box (whatever you call them). I am not sure if i want to do a sq or round one, but it will be 3 tiers. Does anyone know what measurement of boxes I should get? I went looking at michaels and didnt see any so I was going to order the boxes online and i cant visualize what measurements i need

anyone know a store that i can go to purchase the boxes? then i can do it bysite instead of measurements...

tia