Very large bases tend to spawn too close to hills and cliffs causing part of the base to spawn inside the cliff or hill.
It is almost like the plugin is not taking the radius into account when looking for a suitable location to spawn.

this is intended. this plugin does not support very large bases yet and it would require much more thought than just increasing some radius

So what is the Max that one can make the wall radius before this happens?

A fixed value is ideal, which is 20. Anything beyond this requires a distinct calcuation of each radius configured for each position on the grid, otherwise you will still run into issues where bases paste partially into other geometry. This would have to be done when the grid is created, and each distinct radius would drastically increase the time it takes for the grid to load. Doing this on the fly would kill performance on most machines. Therefore it's much easier and practical to use a fixed value instead. You're free to experiment with the value. I'm not against increasing it, I just don't have the patience nor care to change it. I think it's rediculous that people want to paste bases with a filesize greater than 1MB :p

foreach (var position in GetCircumferencePositions(center, 20f, 30f, true, 0f)) // 70 to 30 in 1.5.1​

Thanks. Yes you make a good point regarding the file size :)