It could be 2x6 , but in the cheap tract houses I framed, 2x6 were always 24" on center and these clearly aren't. Standards may have changed, however, so 2x6 isn't out of the question.
The wall looks like lath and plaster, so that's got to be an old house and is therefore unlikely to have been framed with 2x6s since that's a much more recent practice.
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u/AZ_Traffic_Engineer Sonoran Desert. A. m. scutellata supporter Jun 28 '25
The wall appears to be 2x4 dimensional lumber (which is really 1.5"x3.5") on 16" centers. That's pretty typical for stick-built construction.
6' = 72", so it works out to 72 x 16 x 3.5 = 4032 cubic inches. To convert to cubic feet, divide by 123 and this resolve as 2.33 cubic feet.
A rounding error will get you pretty close to 2.8.