Direct mail test group size?

[Al Bunch]

I've read tons of articles on the 'net about doing test vs control mailings. I've taken my list parameters and expanded them as wide as I'm interested in going and that gets me just a hair over 6000 addresses I can direct mail (currently I'm mailing to 2900 addresses).

For those of you that have done test mailings, what size list would you recommend for a test group?