Full bedrock pyramid foundation with lighthouse on top. (Lighthouse itself was used from the Instant Structures Mod so I take no credit for that build, I just changed blocks to bedrock)
Pyramid I build into a mountain ledge (hence the partial underground area) You can do the same or simply fill in the lower parts that it'll create when placed on flat surface.
Complete structure is 75 blocks long x 75 blocks wide x 58 blocks tall
Setting this on flat ground will cause the entrance (and most of the structure) to be on flat ground, but the back will dig 5 blocks down.
Stairway entrance drops 6 blocks down.
Solid glowstone flooring with beacon ceiling
Floor to ceiling is 23 blocks high
Floor is 61 x 61 open area
As of now there is no way to get to lighthouse other than climbing the pyramid from outside. However there is still room above the ceiling to hollow out a small 2nd floor and maybe add a ladder system directly into bottom of lighthouse from inside.