Hi. Thanks so far to the various helpful responses to my queries so far.  I am looking for a spare set of wheels/tyres so that I have a winter set of tyres and non-winter.  My car is a 2004 Legacy Estate.  Am I right is saying that same size (16") wheels from a WRX will fit okay?  

They should fit fine mate as long as the WRX wheels are not from the newer models when they changed the bolt pattern :) 

quite a comprehensive listing for all Subarus when you scroll down

 

http://www.cars101.com/subaru/tiresandwheels.html

i think you could go up to 17's with a lower profile tyre, what you gain in alloy you lose in tyre, if that makes sense

the wheels that were on mine from new would have been 15's, the wheels i have now are 17's, i recently got the 17's off my mates fozzy and he put 15" speedlines on, being lowered i didn't think they would go on without the mudflaps hitting the floor, when i put the 15's against the 17's they were the same height as the low profiles on the 17's and massive sidewalls on the 15's made the equal overall

Thanks folks.  All very helpful.  Cheers.