I would say that 1 opponent on whatever size with highest magic would be toughest.

This is because the computer spreads out so damn quick it's not funny and without opposition it would just go nuts on whichever plane it gets and would quickly spread to the other. Also computer captures nodes a lot quicker than humans do so the magic boost would help it sooner and it's already cheating no upkeep on fantastic creatures boost. Also with only the 1 opponent you couldn't steal as many spells to beef up your arsenal.

OTOH, four mages means that at least one is likely to be maniacal. And at least one is likely to start casting the Spell of Mastery before you're quite ready.

Small land masses mean more time to get established, but they also mean fewer neutral cities to conquer, less good land on which to build, more trouble colonizing, and a potential vulnerability to spells like Wind Mastery.

Since node difficulty scales with strength, the highest-powered nodes ought to be the hardest to conquer. But will give you the maximum benefit (in terms of both power and rewards) once you do.

My two cents; a penny after taxes. ;)