if you google around: mini quilts are to be no bigger than 24" x 24" with small pieces.
doll quilt.. just for that.
wall hanging.. has not rules
I don't think there really are rules if you are not sending your work to be critiqued or in a contest. We all love fabric and just want to sew/quilt.
I sew / quilt small for children's hospital comfort. Not to cover up in, to color their beds, lift the "hospital" aptmosphere.
Good luck.
Ell