I use #0 for standard-sized slabs and #2 for tall boys.
Each allows the slab to fit length-wise in the bottom of the envelope such that it [the envelope] can be folded in half and secured by the adhesive strip.
I have used
http://www.royalmailers.com/ and
http://sunuku.com/Retail.asp
I find that royal has a little better quality (Kraft), whereas sunuku is a little cheaper. I have found both to be reliable.
Scott