You can set any image to fill in the design tool - right hand side under 'aspect ratio'
Select your image layer - then press fill and see what happens. You'll get a warning if the image is too small
Each product max is different and I can't say how many pixels the pillows need
Bigger the better usually (eg 4k square) ymmv
Have a read of the image help files https://help.zazzle.com/hc/en-us/articles/219861927-Creating-and-Uploading-Images-to-Zazzle
