Put this together quick for what its worth.. my oem subaru tire size versus my oem spare tire size. And based off other threads, it looks like Almatti went with a 215/50/17 tire, so theres a comparison between that tire size and the size of a 16" spare tire found in a 2010 legacy. I used discount tire's calculator, found here
http://www.discounttire.com/dtcs/infoTireMath.dos
*Decided to add a 3rd image on the end comparing the size of stock 16" tire for a 2010 legacy, versus the size of a 16" oem spare tire