My preference is to pair BIG girls, the bigger the better.
Usually they start getting paired around 1600/1700 grams and I do not pair female unless they are at least 18 months. Of course the look of the female has to be right as well.
Now this does not mean I will not breed a 1100/1200 grams female either it depends on age and also weight.
For example I will not breed a 1 year old 1500 grams female but I will breed a 3 or 4 year old 1200 grams female.