Envelope sizes for sales letters?

In my last mailing, I use white literature mailers. These were basically just white, rectangular boxes. I received 2 callbacks and converted neither (my fault). The only prospects who visited the specially made web pages I made for them was ONE of the prospects who called me back. In my last mailing, I sent about the same number of letters (42) and use the large 8.5" x 11" envelopes, having almost everyone visit their webpage. It was in a different city in a different state though.

I was wondering what was you all's opinion on envelope sizes and envelopes vs boxes.

I put all my letters in clear report covers with a color coverage to look more impressive.

ysrayl

Imo...

Professional approach: Boxes will stand out and look more important but also cost a bit more.
Personal approach: Hand-written envelope can also stand out and pique their interest. You can try using beige envelopes.