Unless you are going to have pretty small fish, or junveniles in the 10gal QT, then you might say that that size is pretty small. 20L is a nice small QT, but you can go with smaller keeping in mind the fish size going in to the tank. Also, the smaller the tank the smaller the margin for error.
The FAQ Thread at the top of this forum has a good deal of info, suggestions on QT. I prefer to use 100% water from the display for the QT, unless there is something wrong with the water quality in the display. This way, you match your QT water exactly with your display. This significantly reduces stress for the fish. Remember slight changes in water conditions can cause stress in fish....even when the change is precieved to be "better". For instance. Your display tank has a pH of 8.0, but, via buffering, you bring your QT up to 8.3. 8.3 is much better for marine fish generally, but it is not so good for fish who are used to living at 8.0 to suddently go into 8.3---even with acclimation.
There are just so many variables in sea water that we can't even measure as hobbyists. Thus, I say stick to the same water for the display and the QT [when possible and logical]. Its a judgement call, and sometimes you just have to use good sense. For instance, saying what I have said here about my preferance for using display water in a QT, I would never do this in the case of oodinium [saltwater velvet]. This disease is so utterly virulent and deadly that I would place as much distance from the parasite and the QT as I could.