If you are talking about using a protein powder which could cause fluffy bits. I use my stick blender. It does a pretty reasonable job.

But even better, forget the protein powder and use greek yogurt as the base. No fluffy bits.

The smallness of the cut of the fruit depends on how soft the fruit is and how powerful you blending device is. The softer the fruit and the more power the blending device is the larger the fruit can be cut :).

What exactly are the "fluffy bits" made of? The fruit generally works better if it's frozen and in 1" or smaller chunks.