What size box should I ship my Bakugan in?

I’m trying to sell some Bakugan on E-bay, and I wanted to know, what size box should I ship it in? Should it be in a Flat Rate envelope or box? And one’s a B2 and the others are B1, thanks in advance.

Child’s Elmo Costume Size: Toddler Small 2T

Overall Rating:

Total Customer Reviews: (5)
Seller: Amazon
Size: Small Toddler
  • Clothes Size: 1-2
  • Child Height: 3ft 5in
  • Waist Size: 20 3/4in
  • Elmo Deluxe Plush – Size: 3T-4T

    Overall Rating:

    Total Customer Reviews: (5)
    Seller: Amazon
    Disguise Elmo Costume NON-RETURNABLEElmo is ready to celebrate this Halloween with Big Bird, Snufffy and the rest of his buddies from Sesame Street. But, it looks like he’s going to be spending it with your child instead. The Elmo Costume from Disguise is perfect. It is an authentic looking baby costume which is wonderful and looks just like Elmo. The Elmo Costume includes a red plush bodysuit which is great for a cool Halloween evening. The attached character hood features giant eyes just like the real Elmo.