Even up to an inch could be packaged in a slimline box to fit through practically every letterbox. Think how BT do it with their Home Hubs.
Wickham's first Super Smooth were in very wide, slim tubs. I rather liked them:
^ these were over 6" wide and under 1" thick.
Frags and balms? Yeah, flask-like bottles.
I think you're there with your thinking but don't obsess too much about the large letter size. Unless you went for very slim bladders for flat packing liquid/balm there'd be no advantage. Also, remember that a large but thin surface is vulnerable to piercing, and that's not some thing you want with flammable liquids.
Think a little bigger, but still letterbox size.