All 2010-2012 Outbacks and Legacy came from the factory with a 24mm front bar.
2013-2014 2.5 OB/Legacy retained the 24mm front bar.
2013-2014 3.6 OB/Legacy increased the diameter to 26mm.
2010-2011 Legacy GT had a 24mm front bar.
2012 Legacy GT reduced the front bar size to 23mm.
Keep in mind, if the car is "used" to you, it may NOT have retained the factory issued bar. These bars are all interchangeable as far as physical fitment. If you have owned your car from the factory...New, then you can trust the above information. Which tells me, your bar is 24mm.