Build up limit is the same as dig down limit. You can only go 8 meters up or down from he starting point. Unfortunately that means that you aren't going to be able to get an extremely large mountain base even if you shave off the peak and then build up the sides.

If you want a really big base then you need to start some where that is really flat to begin with. That limits you to the meadows, swamps, or the plains. The biome you are most likely to find the largest flattest space to build in with out any indestructible objects in the way is the plains.

I have a 50 meter x 50 meter base in the meadows that only has a couple tiny spots that I couldn't alter. The problem with a base that large is the other side doesn't render and deer and birds can spawn in even if you have full work bench coverage.
https://steamcommunity.com/sharedfiles/filedetails/?id=2860289081
https://steamcommunity.com/sharedfiles/filedetails/?id=2920176358
https://steamcommunity.com/sharedfiles/filedetails/?id=2924127097