This is the smallest butterfly I've made so far!
It measures about 2.5" at the widest point x 1.5" high (6 x 4cm).
It wasn't hard to make this size, but I doubt I'll try going any smaller.
The fabrics are a chambray shirting fabric and a cotton print from Amy Butler. It is fabric only--no interfacing.
You can read more about how to make these here. The fabric pieces for this one are 3" wide x 4" high (8 x 10cm).