When making a submission, at what point (paper count, presumably) does one abandon the tri-fold business envelope and graduate to either a half-envelope, with one fold, or a full-size envelope to accommodate unfolded 8.5 X 11 sheets? Are manila envelopes appropriate for either half-size or full-size, or should one go with white First Class? All this, of course, only after writing & rewriting to the best of one's abilities.
It is to your financial advantage to stuff as much in a small envelope as space will allow - which can vary based on the thickness of the paper but is usually not more then 10 pages. We don't care, honestly, how the query gets to us. We do, however, get annoyed by SASEs that are stationary-sized. That means we have super-fold our reply letter so it will fit and it looks ridiculous. Don't give us a return envelope smaller than a traditional letter envelope.
Try not to go first class unless you have to. (The post office will make you do so over a certain weight, unless you want to send it media mail) It's expensive and it's just a query letter.