I want to buy a donut spare for my Subaru 2014 Crosstrek Hybrid. What size should I purchase?

I would recommend a full size tire comparable to what is on the car when purchased. Why? Most subarus are ALL WHEEL DRIVE. Driving at highway speed will place an uneccesary strain on the differential (axle) with the smaller donut wheel especially the symetrical system! When one tire spins faster than the other side the sensor is programmed to send traction to the other side of the axle "sensing" slippage! This could cause major damage to the differential! This is the reason 4x4 and large SUVs always have a spare that is the same size wheel and tire as the four on the ground! The new Crosstrek definitely should have a Same Size tir/wheel ! If your insistent check the owners manual or contact Dealership for more advice! If this helps please hit my Helpful link! Thanks!