I've got a 30" sump under a 36" tank and it works great.
My skimmer section is 8", which is fine for a small-footprint skimmer with the pump underneath (I run a Reef Octopus space saver 110), but if your skimmer is bigger or has a pump that sits next to it, then you'd probably be better off with more than 8". Mine is also a bit cramped with the filter sock in there.
I only have one 10" baffle after the skimmer section, it goes all the way to the bottom and the water flows over the top into the refugium section, which works fine and also saves some space not having the second baffle there.
My return section is also 6" wide, which works fine with my return pump (BLDC5). I only have a single baffle in front of it, just like your 10" one with a gap at the bottom. I have a piece of eggcrate along the bottom to help keep the chaeto in the refugium section. If you plan to put any sort of mud or anything in the refugium section, then you'd need to keep that extra baffle in there that goes all the way to the bottom.