New pet rat care?

One of my elderly rats passed away two days ago, so I got a new cage mate for my other rats. She's very small and when I went to feed them, she ran away and actually slipped through the cage bars. I just spent the last half hour chasing a tiny rat through my apartment and now, I'm scared my cage may not be right for her. I'm hoping she will grow and become the size of the others, but what can I do to help insure her safety for this point forward?

New pet rat care?
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