For rectangular scarves, try 30 cm x 150 cm as a starting size.  For the super long (25cm x 2.3m) spotted scarf shown, cut two pieces of 50 cm x 115 cm.  Join them (right sides together) to make one long 50 cm x 230 cm piece. Fold in half with wrong sides together (this hides the seam you just made) and serge all 4 sides as before.