The best leaves are from intermediately-matured leaves. Usually from the 2nd to 4th row of the plant. I don't think you'll get hardly any success from a seedling, not to mention you'd have to take a precious leaf off the vulnerable baby. Young, mature leaves off older plants do best. As to the size, whenever you think your plant is strong enough to take losing a leaf in stride.

Here is a link that might be useful: Leaf Propogation