How many birds total? With those dimensions, I wouldn't get more than 5 birds in all, and honestly not that mix of breeds. Silkies tend to get beat up on by production birds. I've had silkies in with my main flock, but it was in my large coop (10x16 or something like that) and large run (16x48). Even then, the little hen got pushed around a lot by the big girls when it came to using the nest boxes. I pulled the silkies out and put them in a separate pen with a bantam Cochin and everyone's much happier.
Silkies are adorable and make great pets. Barred Rocks are pretty, lively birds who really churn out eggs for you. I think it would be best to chose which you're looking for and pick one or the other. If you want the silkies, you could have other ornamentals like Polish or Sultans with them and do well. If you pick the Rocks, you can have different breeds of production birds instead of a flock of all barred Rocks.