I added three new hens (polish, plymouth rock, cochin to replace the ones the predator took) and a rooster (for protection, though some don't think he'll add much) to my three hens (two ameraucana, one orpington). They all came from the same place within two weeks of eachother, so I wasn't concerned about disease, infection, etc. All the hens are around the same age and the size, and the rooster is easily four times their size.
I introduced the four new birds in the evening and had absolutely no problems at all. Added some stress-ease to their water to help with the adjustment.
My theory is that bringing the rooster at the same time balanced everything out. The original birds respected him immediately, and because the three hens came with him they weren't picked on. I panicked for a moment as I thought Big Nasty (the rooster, though he's gentle with his girls) was attacking one of the two hens who have started laying, but chuckled when I realized it was only his version of romance!
I'd read here and elsewhere that you need ten hens to a rooster or he'll torment the poor girls, but I haven't seen that at all. The first night the original three were shy to go in their coop as the new birds had already gone in there, but two days later everybody is chillin'. They're all inside and high up on their roosts before dusk.
I'm new to this and others know a lot more than me, but from my experience size does matter as the new birds will need to be able to defend themselves, and introducing a third variable - in my case the rooster - can help prevent the resident birds from focusing on the newbies.