This is my stackable farm. It has 10 layers for wheat, carrots or potatoes and 1 layer for netherwart. I get just under 10 stacks of wheat from harvesting this. It's all been done on survival, haven't used creative on it.
Biggest farm I made was 8 9x9 squares, for a total yield of 640 wheat after deducing the water block in the center. I could easily make it bigger, I have two large chest full of extra seeds by now, but I wouldn't see the point in doing so. I've already got all the wheat I'll ever need to feed myself and the animals.
Biggest farm I've seen, but not actually built myself was so huge, it took me 3 Minecraft days to farm it all up and 2 additionnal days to replant everything. That farm was simply ridiculous in size, but if you think about it, it's was about the size of a typical RL farm.
After some thinking I realized just how freaking much wheat humans on Earth actually grow. It's an insane amount. It would take more time than a human's lifespan can actually last to farm a cycle of all the wheat being farmed on Earth even if you had the ability to farm it as quickly and easily as you do in Minecraft. 7 billlion people + their farm animals need food badly...