The L6-20 is a 20 amp outlet (hence the 20 on the end), so only requires 20a wiring and a 20a breaker. In fact, it's against code to use a breaker larger than 20a, although you are perfectly welcome to over-wire for possible future expansion.
FYI: I have announced that testing has begun on a prototype upgrade for the upgrade. =) It permits full level 2 charging on the LEAF. It still has the same L6-20 plug and will charge at max speed on the 20 amp circuit.
As I mentioned, It's only a prototype at this point, but my current plan is to offer this as an additional upgrade once rigorous testing is complete. Don't worry, There will be a way for existing rev 1 upgrade owners to get this as well for a nominal cost. I will take care of my existing customers!
Please don't ask anything else, such as when or how much. It's still too early to tell! I will not answer emails, PM's or anything else on this issue until I have more to share. Thanks!
I just wanted to let people know, as this will be the cheapest route for full level 2 charging, and the Nissan/Panasonic unit is by far the best-built unit out there.
-Phil