Havenot got a clue so look forward to an answer.

Who's it for ??? Are they heavily endowed, or not? Take a measuring tape, tie a bow, and check it out that way. That is what I would do. Or go to JoAnns and check out one of their patterns. It will give you an idea, without purchasing one.

'Tipnut' should be able to help you >Tipnut Home > Crafts > Sewing > 50 Free Apron Patterns & Tutorials

The second link takes you to the apron patterns scroll down to 'Dish Towel Aprons' the to No.6 > Full Apron Tutorial. Full instructions are given, the loop for the neck is 18" long, the details given are a little vague. The only thing different that I do is to make a loop on the left & attach 2 "D" rings & make the strip for the neck longer so that it threads through the D rings, that way it is adjustable