See in the chick picture how there is only one that has the yellow "dot" on the top of it's head? That one is the barred rock and that dot is what I was referring to in regards to indicating the presence of the barring gene(s) and how you can see that the other chicks are not going to be barred birds as adults.
The solid birds may be black australorps, yes, as those tend to be very commonly available in the retail setting (farm store, etc). What color is the bottom of the feet on those birds?
As to the size disparity - it is not uncommon to note difference in growth rates between chicks, even of the same breed. As you have only the one barred rock it's hard to say if it is a breed difference or just a matter of her being a smaller/slower growing bird that would seem smaller even in comparison to others of her same breed. Where does your barred rock seem to fall in the pecking order? Do you have more than one feed/water station available to them to prevent domination by the larger birds- sometimes when a size disparity develops the larger birds can dominate resources like feed and water and this can make the disparity grow as time passes and the larger birds get more of the feed and the smaller birds less.