Made mistake in interview, fix it in thank you letter?

Hi, today I had a great interview. But I have a strange little issue that I need advice on. The company I am going to is rather large and the employer asked how many people were in my current dept. I fumbled a bit and couldn't quite remember, and in my nervousness I told the employer how many were in my subdept. It turns out though that overall my entire dept is comparable to the that of this employer. So, my questions is should I clarify this in my thank you letter or do you think it doesn't really matter. (She asked me on the fly). I am just concerned that she might think I am coming from a tiny little place and going to this big one which might suggest a weakness in my fit there. What do you think? Should I draw attention to this in the thank you or not?