One of the least visually appealing aspect of the Highlander / RX330 series of SUV's is the ridiculously tall and skinny tires. It makes the vehicles appear top-heavy and ungainly.
I took delivery of my Highlander Hybrid four months ago. One of the first things I did was upgrade the wheels and tires, and had snows mounted on my factory wheels.
For summer, the vehicle rides on 255/55 18 Pirelli Scorpion tires, mounted on Voxx aluminum wheels. What a difference!! Not only does the vehicle look better, but it handles *significantly* better than before. Quick turns (and therefore emergency maneuvers) are handled with ease.
For winter, I have 235/65 17 Michelin Pilot Alpin tires. So far we've not had much snow here, but the tires have handled what we've gotten with complete ease.
So to obliquely answer the question about using different sizes, the two sizes I listed above have had no problems with fitment or performance.