What are the dimensions?

I do not know the actual dimensions. It is an industry standard power supply, so it will work in pretty much any industry standard case. So long as you are not trying to put it into an odd ball Dell or HP case, everything should be just fine

Cant tell from the pictures, but does it have dual S1 (4 pin) connectors for the MB? I also assume it has at least a single 6 pin for video since it is SLI compatible?

in short: yes. the 4+4 CPU connector is semi-visible in one image, but i've had this psu since 2014, it is there, and very happily runs my ATX mobo. For SPI, it has 2x 6 pin, and 2x 6+2 pin, so it can supply a total of four graphics cards, if you were so inclined (however, 800W likely wont run four cards)