There's a wise old saying in regard to breeding that applies to most animals.The pattern (mottling) was low on their priority list as long as some of them retained those genetics.
"Build the barn before you paint it"
The size is primarily to evade smaller predators.
Unfortunately there is no chicken large enough to ward off even the smallest of determined predators. A measly one pound weasel can easily wipe out a whole flock in no time.
While color can help to camouflage them once hidden, many predators actually detect prey by movement more so than any particular color.
My ultimate goal is a large dual purpose breed, dark or mottled, that goes broody (MJ and JJ), with a good feed to meat ratio (BA) and OK with heat (MJ) or cold (JJ and BA). I want them to mature relatively quickly (BA), good foragers, and produce eggs through the winter (JJ and MJ).
If you are at all relying on hatchery descriptions... don't! Unfortunately they are notoriously inaccurate and exaggerated. Anything relating to broodiness, size, meat and egg laying abilities should be taken with a grain of salt.
Since you are creating a landrace I wouldn't even worry about breed at all and would focus more on picking outstanding individuals that are close to your ideal. You may have to order multiple batches of various breeds to grow out and select from in order to get what really want. Best of luck with your project!