I wouldn't mess with the tire size unless you get a screaming deal on something close to the stock size. The CVT can already lower the engine RPM below what's required to maintain most speeds, so you would probably just be adding more weight with taller tires. Here's a little explanation of how the tranny works (I believe it's for the Gen1 Prius, so the exact numbers and ratios are probably different for the Gen3).
Toyota Prius - Power Split Device
Financially, running the current tires till they need replaced would probably be best. You'll have a difficult time getting decent money out of the used tires, and the Prius is so efficient/gas so cheap that the slightly lower mileage won't make much difference. If you're just looking to put up big MPG numbers, then by all means, swap the tires.
