25x25 shouldn't work.
If you look at the SI stats on Stone, truthfully you should have support every 7 blocks.
[support, then five spaces, then another suppport]
That's with one layer of block above
That changes slightly with 3 layers [after that doesn't matter.]
If you're looking to build underground might I suggest making a test game.
Build in creative with stone and see what it takes to make it fail.
This is how I cam up with the design of having a support column, five spaces, and another column.
Never a single collapse even with a very large build above.
At least this answers my question about how people are getting so many collapses in A17.
I think it's a lot more accurate as far the the SI stats go than A16.
In other words, it's working as intended.... I suppose?!?!?!
If you want something larger and don't mind modding in some stuff try adding a recipe for
Riveted Bridge Metal
Bridge Concrete
Either of those will give you the longest span while supporting the most weight.
This might help
The stats are still the same for those blocks.