27.5a is the max a current Leaf will charge at, anything more than that is ignored and will only charge at 27.5a.
AFA your 240v service, personally I'd have a 14-50 outlet installed, 2 hots a neutral and ground. True you don't need the neutral for a EV but if you ever need it, it could come in handy. RVs use 14-50 outlets as well as stoves. Of course you don't need a 50a circuit with a current Leaf but it shouldn't be that much more to wire and you'd be futureproofing yourself, a 14-50 is a very common EVSE plug.
Personally I probably wouldn't purchase a EVSE much larger than 30a, going above 32a will result in much thicker and heavier J1772 cabling, a 10 gauge cable is hard enough to work with, 8 gauge is even worse.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a