Most of my animal blocks look great on pillows, but not all of my patterns include complete instructions for making pillow covers.
To make adapting these patterns easier, I've made a "cheat sheet" with basic guidelines for constructing 18" x 18", 20" x 20", 24" x 24", 26" x 26", and/or 16" x 26" quilted pillow covers.
This document describes the basic process of making a quilted pillow cover with a bound envelope back. You may need to do a little bit of calculating to figure out how much fabric to add to a block to make the desired size pillow front, but the document includes all of the yardages and sizes you'll need for the pillow back panels, batting, and lining for each pillow size.
When selecting a pillow form for a quilted cover, I usually go with the exact size of the cover (for instance, a 20" x 20" pillow form for a 20" x 20" pillow cover). I prefer to use down or down-alternative pillow inserts from IKEA or other home and furniture stores instead of the stiff poly types that craft stores usually carry. The insert in the Bjorn Bear pillow pictured above is from IKEA.