Personally, I tend to distinguish between a QT and a hospital tank, but I see many hobbyist who do not make that distinction which is why I suggested thinking about a larger tank.
If the QT is only going to be used to house a newly acquired specimen prior to introduction into the main setup, then the 10gal should be fine for most fish. However, if you are planning on using a QT as a hospital, then I'd go with, at the very least, a 20L. It all depends, however, on the fish you have in your tank [and the ones you plan to have in the future]. If your fish are mostly smallish, then 10 might fit the bill. However, as Ed's exp demonstrates, larger fish may become serious predators when placed in close confines with placid tankmates.