I guess it depends on which Kermit you look at, but from the pics I've seen, it's an almost completely flush join.
As for the body pattern, I looked at a few other people's patterns and resulting puppets and made my own from what I'd learned also made my own collar pattern. In fact, I don't know if there's a single piece I've not modified in some way. I've had a good deal of trial and error to get to where I am though. The body pattern I ended up using is from the 2nd one I'd drawn out.
My Kermit's got a 24cm collar, so each of the 4 body panels is 6cm wide at the neck. It's somewhat pear-shaped, and leads straight into the sleeve. With a little guess work and eyeballing, I decided the belly would be about 52cm in circumference, so the widest part across each of the panels is 13cm.