How would I find out what size trailing arm spacers I need for the lift i have? I have an ADF lift on my 98 Legacy Outback, I believe it's 2" (I'm not the original purchaser of the lift). Should I just measure the gap between the front of the rear tire and the fender, then compare it with the rear of the tire?

Measure whatever lift block you have on the subframe or above the strut. 

I believe you can get away with removing half an inch from this measurement and use this to fabricate the new trailing arm mounts. 

If you can make your own mounts, you have the advantage of making several sizes to see what works best for you and keeping your costs down in the process.