Hi Laura,
If you select one of the new in-built corpora, AntConc will show you a table of information about each of the corpora, including the size and reference information. If you want to read more about the corpora, please see the reference there.
As for your question, the AmE06 is a US general English corpus and the BE06 is the equivalent British English one. These are one-million word corpora, which are suitable for general studies, but you might want to build a bigger corpus from raw files yourself. I'm looking into adding the BNC to the list of available corpora, but I'll need permission from the owners before I can do that. If you find a good corpus and think it would be good to add to AntConc, let me know. I'm hoping that the AntConc repository of corpora can grow, making it easier for people to do research such as you are doing.
I hope that helps!
Laurence.
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Laurence ANTHONY, Ph.D.
Professor of Applied LinguisticsFaculty of Science and Engineering
Waseda University
3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
WWW:
http://www.laurenceanthony.net/###############################################################