Kyiav,
I think Wendy has it right. You're going to have to do some measuring to figure out what size width to make the body. Measure the finished width of the wings and plan accordingly. It looks pretty narrow so probably cut an inch wide strip that would become 1/2" finished.
If you're making several butterflies I'd make a strip of fabric with the top and bottom sewn on and then cut the bodies from that.
I'm really surprise Marcia doesn't have the body measurements posted there. She's usually very precise. Perhaps drop her a note and ask what she planned to use there.