Mine is a 5EAT sedan, so it's a bit heavier than a MT sedan. Similarly I think there would be differences with the 5EAT wagon and MT wagon. I used 3/8" but the vendor that I got them from (Mach V) makes them in 1/2" and 1/4" too I think. So it depends on which Legacy you have. I've seen pics on this forum of people with spacers and still had the saggy butt on the rear struts.
Also, the type of Pinks you use on the rear struts will be a factor. I opted for sedan/5EAT springs all around, but I know some use the wagon springs in the back just to avoid the saggy butt.
Just to note, I recall doing a search here and it said that if you use 3/8" spacers you did not have to use extended studs. I actually found that this was NOT the case and I did have to use extended ones. I don't know if this was just on my car or what, but the spacers were indeed 3/8" thick. Also, the stud holes didn't line up properly and I had to enlarge them somewhat to fit.