Good morning, John~
I have thought of making Hens a bit smaller than Drakes - but never have. If I did, I would have my Drake plans reduced on a copy machine where you could dial in exactly which percentage you want (90 %, 95 %, 93.5 etc) to maintain proportions. I have thought, too, of making the Hens' bill a tad shorter than the Drakes.
Although the average (mean) Drake is indeed a bit larger than the average Hen in most species of ducks, there is overlap between young birds and adults, etc.
Bellrose Ducks, Geese and Swans of North America gives these average overall lengths:
adult male 24.7
adult female 23.1
juvenile male 22.3
juvenile female 20.4
Extremes run from 19.8 to 27.5
So, the adult Hens are 93.5 % of the adult Drakes - roughly one/sixteenth smaller.
Let us know if you follow through.
All the best,
SJS