Interesting that you picked 50x50.
I generally use block strength to determine my exact size.
If you're using creative or a modded server then you can use
Riveted Bridge Metal
Bridge Concrete
[both hold 400 mass]
With those you can build with spans of 9 blocks [provided you put in support at the top to make it a span of 7.
Anyway, that's how I design my place.
I make my foundation based on how far my support beams can reach and that's how I move forward.
If you're strictly Vanilla your spans should never exceed 7 and you should have support block at the top as well to help.
Click this link
Working-Castle-Tribute-to-A16-Plus-bonus-tips-video
The second video [you don't have to watch it all] I talk about SI and stuff.
You probably know all that stuff anyway but I mention right near the start spans and supports.
That's how I build anyway.
I know the rooms I want, I just don't know right off the bat where I'll put them.
I just mark them out with frames and kind of see how it goes.
Have fun!
Large bases are worth the effort.