I'd guess that the pan should be about 12 or 14 inches across, and the sides of the pan should be fairly high, maybe six inches. Mary looks for a mathematician around to check with. I don't have one, I find a cake tin with an empty baked bean tin in the middle and pouring the mix in carefully works fine. It is unavailable in Australia. Do not substitute cooking chocolate - this has sugar in it.
Semisweet chocolate is more like the dark chocolates available in Australia.
Use any dark chocolate. Dark cooking chocolate is just fine. Milk chocolate is too sweet, don't use that. Oh dear, this one is complicated. In other words, green onions, scallions and Australian shallots are pretty much the same thing.