I have always been told size doesn't matter hehehe

Long morning without coffee...

It is giant scale, however by today's standard the small side.

Anyway the 80" will fly better than the 72" if built comparably well. You will see some increase in stability if kept light.

The techniques will be similar just more room to work with. You won't see the elaborate building techniques used until these planes start being built for competition. As light and strong as possible.

As far as engines go the gas engine will cost more up front, than you only pay the pump price for fuel plus oil..

Dual batteries are not necessary for a plane of this size. However a 1200mha minimum is something to think about. I also prefer 6.0v for this purpose. And possibly ignition battery and switch...

Dual servos on the ailerons and elevators, optional on rudder (good heavy duty will suffice)I have flown many 80" birds with s148 on the rudder, with out problem. However the more extreme requires the better servos.

As far a field that depends on the pilot and comfort level. If kept light depending on plane it will probably slow down enough, yet I haven't seen the field nor the pilot so its your call...