Depending on your greens you don't need much of a cup. We grow lettuce in 2" net pots.

I would advise against growing leafy greens in 5 gallon buckets. I could see one plant taking up the entire top of the bucket. I would spend 12 bucks at home depot, get a 27 gallon tote and plant multiple plants in the tote with small net pots like Rio Grande mentioned. Just my opinion though.

I have grown with small cups in big totes and I found that moving the tote became impossible when it was full of water. I also found that the roots would tangle making it hard to remove plants so they could be replaced when they were done. I was hoping to use multiple buckets to make them easier to move and hopefully by putting identical plants in a bucket they would all get replaced at the same time.
I grow outdoors and I also struggle with keeping the plants from tipping over when it gets windy. Maybe I need to cut the holes to fit the net cuts tighter but I was hoping to get a more stable setup by using larger cups.
kevin

I say keep it simple I grew some asian vegetables in a small DWC no problems, easy to handle.
I have another two larger DWC with 10 holes fully planted no problems.
PS in the photo posted I did not even have an airstone, and the plants grew beautifully.