Subaru's have very limited wheel well clearance so I would not go any bigger the stock tire diameter. You can use an online tire calculator to pick the right tire size.
F_O_R's correct in that the tire diameter is already pretty tall for the wheelbase of the modern OB. To improve handling a bit Subie specs 18" wheels for the Limited, but even with 60 series tires on 18's the body roll, dive and yaw are borderline-sloppy, so I don't think going to 55 series tires on 19's will do anything except drain your wallet. It's kinda sad....
From what I have seen the Outback seems to be aimed at the over 50 crowd that wants a cushy car. In my area new Outbacks are pretty much all owned by the geriatric set (no I don't think 50 is old). ;)
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