You're an idiot. If you're new to drifting, you shouldn't be moving into a 350 hp car when you probably couldn't even get down in your 240 hp one.
Like already mentioned, there is no ideal tire size for drifting a 350 hp car. Tire size is not equal to grip.
There are several factors that effect grip besides power (i.e. speeds, vehicle weight, suspension geometry, track conditions, tire compund, pressure, temp, etc...)
With that said, 17's in 225 to 235 are pretty decent for the rear, plus, they're cheap and available.
__________________
Holding it and...WHEEL POPS OFF! O^O^O
|