Kettlebell sizes - women
I recommend women under 60 years of age buy one 8kg kettlebell, one 12kg kettlebell and one 16kg kettlebell.
I recommend women 60+ buy one 6kg kettlebell in addition to one 8kg kettlebell, one 12kg kettlebell, and one 16kg kettlebell.
Kettlebell sizes - men
I recommend men buy one 12kg kettlebell, one 16kg kettlebell and one 20kg kettlebell.