Direct Mail Envelope/Paper Size

[Brock Mogensen]

I was wondering what the best envelope and paper size is for direct mail.  I was thinking an A6 size envelope with A6 size copy paper.  Has anyone found better success using postcard thickness A6 paper compared to regular A6 paper?  Or should I use a bigger size paper?

I sent out my first batch of direct mail today in regular business envelopes with a standard paper size.  I've read all of the posts about direct mail so my message and font were clear.  The blogs just didn't talk to much about what envelope and paper size they use.  

I appreciate any advice!