You can grow quality weed in a Solo cup, just not a bunch of it. Personally, I think solo cup grows are killer. There is a user on RIU that was the winner of High Times Top Hot Pot Shot of the Week using beer cans and CFL's. He pulls 4 oz's a month doing a SOG style grow. His grow pretty much shoots down the idea you can't grow quality weed in a small container.They look like they should be transplanted soon, I understand you would like to do an experiment to see what you get, try at least 2L. Compare 1 gallon and 2L if you want to experiment. Other wise the solo cups are going to be very disappointing, Even if you do not care about your amount at harvest, you should care about the quality imo, and show the beans a little more respect lol. They wanna grow, let em grow!
edit: People do wonders with 2L hempy SOG check them out