Originally posted by Oakshield:

If you want a small base, find the location of the Queen.
When going up the stairs, look for a small 2x2m wooden wall piece. Most likely found at the left side, if going up the stairs; about 4m inwards.
Destroy that wall piece and look behind it.

There's enough room for a portal, a workbench, a bed and most likely a fire as there are some openings available.

Thorin :)

I don't know what is typical but after wrecking all the unnecessary black marble blocks and digging a bit with the pick mine was probably a 15-20 M radius cave. The default infrastructure uses a lot of those really big blocks and none of it is needed to support the stairs and alter/entryway. If you do need support, a bit of ironwood at an angle can keep the stairs up without filling the whole space.