As for an envelope size, I would suggest the size of envelope used for invitations. Sizes vary, but are generally larger than the #6 standard cover. Here's a link to
Amazon showing some of the varieties available.
https://www.amazon.com/s/ref=nb_sb_...+invitationsAs to your question about a cachet, rather than paste on a photo, have you considered printing a photo image onto the envelope? I recently did a couple of hundred covers for the new US Airmail stamp printing the cachet onto the envelopes with my inkjet printer. It took a few "wasted" copies to figure out how to lay out the image on my screen to get it to print where I wanted to on the envelope, but after that it worked fine. If you have a paint/image program with the ability to create a "vignette" in a sepia tone that might go along with the occasion.
Basil