Imi has asked me to post a few suggestions as to how big a quilt should be, so here goes:
Quilts intended to go on a bed usually have an overhang of 8-12 inches on each side and at the foot of the bed and reach to the headboard. Thus the size of a quilt is dependent on the size of the mattress. The table embedded below shows suggested sizes for the various sizes of mattress commonly available in UK and doesn't always match standard duvet sizes.
I have used imperial measurements rather than metric because most patchworkers use imperial measurements.
Newborn babies usually sleep in a pram/moses basket size crib and are best covered with light blankets rather than a quilt, but a quilt may be used as a playmat placed on the floor. The size to fit a standard cot, 36" x 45" is a suitable size for a playmat.
Lap quilts can vary a lot in size. To cover legs and feet whilst sitting in a chair, 36" x 48" is big enough, but to snuggle right underneath 54" x 72" is a popular size. Remember these are only suggestions, a few inches either way is OK, especially with the bigger quilts.
Wall hangings vary tremendously in size; it depends on the available wall space. Just make sure there is a small amount of wall left all around the quilt when it is displayed.
Cushion covers are commonly 15" x 15", 16" x 16", 18" x 18" and 20" x 20", and remember that the pad inside should be 1-2" bigger so that the cushion is nice and plump.
Standard UK pillow cases are 20" x 30".