Like any other garment, the size of a scarf or shawl depends on personal preference plus the yarn and stitch pattern used.
Following are suggestions for an (adult adjust for children)
Scarf
Length:
Fingertip to Fingertip
60" - 80" (152 - 203cm)
Width:
8" - 12" (20 - 30 cm)
Shawl
Length:
Fingertip to Fingertip
60" - 80" (152-203cm)
Width:
30" - 45" (86-114cm)
Shawls can be knit in a wide variety of shapes, rectangle, triangle and everything in-between!