Hussar
Legend
Have you considered making them decide quickly? Give them like a count of three to pick the center-point.
That might be one option. But, it tends to be a bit tricky to implement in game.
I wonder if adding a couple of checks to casting area effects might not do the trick. Say an easy, medium, hard caster check for placing the spell. 20 feet is easy, 100 feet is medium and anything over 100 feet is hard. Miss the check and roll a scatter die - 1 square off for short, 2 for medium and 3 for long. That at least makes spell casting less about solving a geometry problem and more about actually playing.
A second caster check might also work to vary the area of effect slightly. Fail the check and the spell (50:50 chance) is either bigger or small than expected. Maybe DC 8+spell level. Add or subtract 5 feet (minimum 5 feet) for every 5 you fail the check by. IMO, this would go a long way towards making magic a little less pedestrian and a lot more exciting.
Then again, perhaps simply going with 4e here and all effects are square. Greatly simplifies the whole issue and stops people from doing the whole, "hey, if we shift the template by 5 degrees, we'll hit all the orcs and not the fighter" dance every time an area spell gets cast. Yeah, yeah, I know, square fireballs and all that. But, it does have its benefits.