The way I dig out anything of any size, 100% safely:
Only use one miner. One dwarf in the entire fort should have the mining labour enabled. Only one. Just one. One.
If on the surface, first cut down all the trees in the area you want to hollow out/dig out.
To start, pause the game.
Next, at the top layer, designate downwards stairs in the entire area you want dug out.
On the bottom layer, designate upward stairs in the entire area you want dug out.
In between the top and bottom layers, designate up/down stairs in the entire area you want dug out.
Now, unpause the game let the miner dig all that out.
To make it hollow/air where all the stairs are:
Starting at the top layer, designate channel on all the downward stairs.
Once that is done:
On the next layer down, designate channel on all the up/down stairs.
Once that is done:
... repeat until the bottom layer. ...
Once that is done:
On the bottom layer, designate remove stairs on all the upward stairs
What happens is, after the first level is channeled out, all the rest of the channeling is done from below. With a single dwarf doing the work, there is no chance of one dwarf excavating where another is working. (which is how fatalities occur, even using this method, if you have more than one dwarf with the Mining labour enabled.)
I have used this method dozens of times in very large projects and have yet to lose a single dwarf. If this is unclear, I'll provide some pictures.