Video Tutorial

To know the sizes you have t know:

Witdh: From elbow to elbow
Length: Contour of the arm
 

Make a rectangle of the sizes that you have following the chart

Chart

Sew the parts in green to make the armhole

Edge

On each chain space of the neck and the back:

-1 treble crochet, 1 bobble stitch, 1 treble crochet, 1 bobble stitch