I bought some "Social Envelopes" at Walmart. Size 5 3/4" X 8 3/4". This is a standard size for the post office and will go for the standard rate, up to 3.5 ounces. I am sure it will weigh more than that, but it shouldn't be too bad. The thickness of your envelopes have to be under a quarter inch to be considered a letter, not a parcel. I am thinking that this size with only one fold in the fabric will easily be thinner than that.
Social Envelopes comfortably hold and 8 1/2" x 11" paper, folded in half. A fat 8 cut is 9" x 11". I think I can make that work, though I might have to shave off a half inch on the short side.