Kettlebell sizes - women
I recommend women under 50 years of age buy one 8kg kettlebell, one 12kg kettlebell and one 16kg kettlebell.
I recommend women 50+ 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. Men under the age of 50 will also need a 24kg kettlebell.