I imagine her to appear around 15 or 16 since she estimated Josie to be 14 and a half and she said that Josie was shorter (until the last chapter where Josie was a little taller than Klara).

Then again Klara might not be as heavy as a real person, making it easy for Rick to carry her. She was supposed to ‘continue’ Josie but she was going to get uploaded to the body that Mr. Capaldi was creating so her physical appearance didn’t really matter.

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I think around 16 makes the most sense but it’s hard to confirm