That's a good place to start yes, I've always found that being very selective with the females in particular will make the biggest difference. The females will weigh heavy in the type of the offspring. I notice a lot of people focus on the males in different breeds and are more lenient with the females, and that might be because they are perceived as the easier of the two to work with, and in some ways they are. What you will want to do is select your widest and best built females onto your largest male you've held back and do some test matings. I always tag mine separately so can watch their development, this will be my third year working on the females intensively and last year had several females that were bigger from hatch, clear until about 2 months of age, then the boys finally started passing them. Those same pullets are now some of my best stock, including Luella, she was almost twice the size of some of the cockerels at hatch. In fact, everyone in my family and friends that saw her when she was little all said cockerel. After about 4 or 5 days tho, I knew I had a pullet on my hands when she was growing some crazy wing feathers and had a tail already.
There are other tips to working on size, particularly if you are limited in your options, but size and type on females is an area where I won't compromise as narrow or smaller females can slow down progress quite a bit.
