The industry standard (for what it's worth) is generally 20" - 21" Seat height and 20" to 22" in seat depth. Of course, one size does not fit all. What works for good napping and lay-down does not translate into a good sitting sofa. I personally think this is one of the best all-around sofas I have come across, though admittedly the style is not as current as some pieces and it's fairly plain. This is a workhorse piece however that I think is great for naps and for sitting for most people.
http://www.hancockandmoore.com/Produ...ns/Kodiak-Sofa
For more transitional pieces, I (and my wife) really like this sofa, has a lot going for it AND is in the Just in Time program.
http://www.hancockandmoore.com/Produ...Sofas/Ava-Sofa