You can improve size by crossing to other varieties. Some of the blacks are getting some good size and these could be used for an outcross. You would still have to breed the F1\'s back and then the F2\'s back again to get to your original color.
There has been a lot of discussion lately with some other breeds that are improving size just through selection. Buckeyes are what I have read the most about. Hatch a lot of birds weigh each of them at twenty weeks and keep the biggest males and the biggest females as your breeders and get rid of the rest. Use the front section of the APA Standard as your selection guide. Select not just for weight but back width, abdomen depth etc. You should see improvements within a couple of generations. Probably as quick as you would by outcrossing and then breeding back to the variety standard.
One thing I would suggest when doing this, Cull for weight, width depth FIRST. Then cut the remaining birds in half again culling for breed type and color.
Only plan on keeping about 10% of what you hatch.
If you cull for color and breed type first, it will take you longer to get the size you want.
Curtis