I built my oven a little over a year ago, and after a year of much use know what I would keep, and what I would change. Rather then bore you with all the details, I'll get to the meat of what I would do if I was to do it again.
1. Follow the fornobravo plans for materials and methods, but not shape. I used homebrew mortar and perlite to insulate, saved a ton of money and am happy with the results of both.
2. Keep it low. My dome is 13.5" and at 900F+ on the hearth I feel it gives a good balance of heat on top without ever having to sky the pie. The rule of thumb h=d/4.3 seems like a good one for ovens with diameters in the 40's(inches) which is where you probably want to be. If you anticipate wanting to cook a more american style pie at 700-750F on the hearth you probably want to go a little higher so you don't scorch the top of your pie.
3. As already noted, it really sucks to cook too close to the coals. A 40" or 42" gives plenty of room and even allows for a little bigger door if you want one.
4. Keep the mass where you want it, in the oven. Lots of ovens, mine included, have large masonry mouths and transitions leading to the flue. All this mass out in front of where you are cooking is robbing heat from your oven. If I were to do it again it would be much more like you see on the ovens built in naples, a vent cantilevered out over the door with no mass bellow it supporting it.
5. Vent the oven back over the top of the dome, then up. This will increase the draw. By being proportional to the oven height, the lower the dome you have, the lower...and smaller the door. I feel my oven draws pretty well, but sometimes I still feel like I am a little starved for oxygen. A draft door makes a huge difference on my oven, but can't be used while cooking cause it gets in the way. If I had vented back over the dome I don't think any of this would be an issue.
Basically what these leads to is an oven based on the ovens built in naples based on hundreds of years of experience, using modern materials that are easy to get here in the US. It is by no means an authentic oven, but would be the cadillac of backyard home built ovens in my book.