Does this link help? I only clicked on a couple settlements, but it seems to include the sizes for the ones I clicked (e.g. "large town"). Edit: I tried a few more, not all of them have sizes listed. But I think it's the best you'll find on the matter. If you have a map, I'd recommend to guess sizes based on location, like nearer to a road is likely larger, that kind of thing.
Also, you may find my PFRPG Random Item Generator useful. Select a settlement size, and click Generate! Uses CRB for the number of magic items of each degree of strength in a settlement, and UE tables for the actual items.