The 3D view on United's website doesn't really give me a good indication, and the Wiki is slightly outdated.
How much clearance is there for putting bags under the bulkhead wall when sitting in row 7? Enough for a slightly bulky messenger bag? As perpetually being in Boarding Group 2, I actually wonder if Row 21 is better since the odds of getting decent overhead bin space near row 7 will probably be slim to none after half the plane fills up with Boarding Group 1 folks.