I've started working on a 100x100 castle. I laid down the outline just to get a sense for how big it's going to be. I think I need a lot
There are two mountains I have to level out, and one corner of it hangs out over a beach. Sadly, this was the flattest area I could find anywhere near my spawn point. I have enough iron I can use iron pickaxes and shovels for everything, though. Helps move things along more quickly.
I haven't laid out the floor plan of the castle yet but it's big enough I put whatever I want in there. I've been meaning to create an underground tree farm, so I'll probably build that underneath.