high flyer you can use a 2S lipo or 7V with the stock 400 motor and it will perform well
a 3S will make a short life of the motor if your not very carefull on the throttle, some will argue this but I have cooked my share of brushed motors on 3S
as to size that realy depends on how long you want it to fly and if you want to have to add weight to the nose to get your CG right
also you will be limited by the batt bay, however putting 2 batts together in series to double your ma works well for making up needed weight
I think 1300 or so is going to be about the right size for the plane