the 16x16 farm is more efficient with about 7 floors or something like that. I recently posted pretty much the same exact question, and I have more answers, I'd take a look at it ;)

 

16x16 13-18 floors (auto is good but the 16x16 should be in a chunk

 

true with a 200x200 wheat farm u can harvest at max 39204 wheat in one harvest (yes i realy went to the trouble to get that)
but it wouldnt be efficient because u got to plant all those and collect them after auto harvest which is near imposible without a ton of people working for you. not to mention the massive lag when u trigger it

i would suggest a 16x16 light farm.

 

16x16 perfectly on a chunk at least 7 floors tall is efficient enough, having more floors than that is not really needed but can help in case you fill quickly.

 

The height really depends on how fast you harvest, of your uber leet fast then more floors are needed :D if you move extremely slow and afk every 5 mins then less floors are needed

 

So a chunk is just a 16x16 plot?

It's not like you have to "Find" a chunk? Before you can start planting?

 

You have to find the chunk first.

 

You can use a mod to find chucks or you can divide the x and z axis by 16 till and even number (no decimals) comes out, then you just count 16 blocks to find the other corners. To double check the cornners are correct stand on the corner (make sure your standing on and not next to the corner) and divide the x and z axis again by 16.