We also have a 2012 Forester Premium w/ 17" wheels and a manual transmission.
However, the base trim model was fitted with 16" wheels and 215/65R16 tires. Accordingly, when it comes to a full-size spare, you have a couple of options:
1) A 17" wheel with a 225/55R17 tire, or
2) A 16" wheel with a 215/65R16 tire
Our f/s spare is the 16" wheel with a 215/65R16 tire; 17" steel wheels were not readily available when I purchased our f/s spare in 2009. Our winter tire set is also 215/65R16 tires on 16" steel wheels.
HTH,
Jim / crewzer