My understanding of a fidget quilt is just made of squares and/or rectangles using textured fabrics like faux fur, silk, velvet, polartech, satin, velour, etc. The folks just stroke the fabric and feel the textures which I understand is very calming. Its almost like the "blankie" that babies use.
So just choose a block size and make a laprobe. I think texture is more important than size.
Hope this helps. Suzanne