According to the view of Rabbi Avrohom Chaim Noeh in Shiurei Torah the following measurements regarding challah apply:

1. Do not separate challah when using flour less than 1.23kg (approximately 8 cups of unsifted flour or 9 cups of sifted flour). 

2. Separate challah without a brocha when using flour weighing between 1.23kg and 1.67kg (between approximately 8 to 12 cups of unsifted flour or 9 and 15 cups of sifted flour). 

Separate challah with a brocha when using flour weighing more than 1.67kg (approximately 12cups of unsifted flour or 15 cups of sifted flour) . 

A more strict opinion rules that a brocha on taking challah should only be made if the flour used weighs more than 2.4kg.