The slab was poured using 5:1 ballast:cement in a 6"x1" wood frame, and was originally sized to take a arched style brick oven as detailed in Alan Scott's book (see the links). Then I realised how much the firebricks would cost me - they seem really expensive in the UK compared with US prices. So plans changed... I'm now planning on building a round, domed oven and haven't decided on whether I'll build a firebrick or a earth/clay dome (adobe/cob). This will probably depend on whether I can get any cheap firebricks before I need to start building the dome. Here's the slab, all marked out ready to go. I'm only building within the big circle, so my maximum diameter will be 1550 mm. Hope that's enough space - we'll see.