It is true that the panel will change in length, but the wood will change as well, so it is the delta between the material thermal expansion coefficients. Looking at the values for fir and steel it is about 3.7 for fir and 11 for steel, so the delta is reduced to 2/3 (ish) of the value you find, so that 3/4" becomes 3/4*2/3=>6/12=>1/2" and 1/2" becomes 1/2*2/3=>2/6=>1/3"

And that amount is spread over 60ft=720" so per foot is 1/2*1/720=> 1/1440" per inch

The fasteners are designed to handle small movements...