Do you have a pizza peel yet? Get one, and measure its horizontal width. This defines how wide you're going to want the opening of your oven.
Our favorite peel is 21" wide, so we wanted our oven opening to be 22-23" wide. Again, it's one of those things where smallest is best, because of heat efficiency, but it does need to be wide enough to get the peel in and out with ease.
TIMING IS IMPORTANT HERE -- you need the adobe dome to be dry enough that it won't collapse when you cut into it, but still moist enough to let you cut into it (truly dry adobe is like concrete).
When the adobe dome has dried a little -- the outside should be leathery but not sandy-dry yet, about 24-48 hours after the adobe dome was finished -- use a stick or pencil to trace out a mousehole-shape in the front of the adobe dome.
Now use a steak knife to saw into the outline of the mousehole-shape (including the line at the bottom). Be sure to cut through the thickness of the adobe layer -- the knife will plunge into sand on the other side of the dome wall. Slowly and gently pull out the mousehole-shaped adobe and put it to the side.
Using your hands, reach in and gently dig out the sand from inside the oven. Do this carefully and slowly. When you feel the newspaper layer, you know you've gotten all the sand away from that section of the oven. Keep removing sand until it's gone -- the oven will be hollow.
Give the oven a few days to dry. Then build a small fire on the lip of the oven and when it's going, shove the fire inside the oven. Keep adding wood (kindling-size) to this fire throughout the day to keep a low fire burning a long time -- this will speed the drying-out process!