You got it backwards. AGE is more important than SIZE. If you have a 1200g female who is only 14 months old, that doesn't mean she's ready to go. If you have a 5 year old female who is only 1100g, then by all means go for it. I wait for 3rd winter and 1500g+. If she is a long, skinny 1500g, I wait. If she's a shorter, chunky 1500g, then she's good to go.

If she's only a year and a half old, just wait. It's not worth it to stress her body when she's not even done doing the majority of her growing.