For the record, this is the description from the photographer at Astrobin
I took this image over a year ago, on 12 January 2022. I used a Samyang 135 mm lens @ f/2.8 and a ZWO ASI 2600MC camera (gain 100, bin 1, -10 °C). The setup was mounted on an SW AZGTi mount operating in equatorial mode and was controlled by ASIAIR Pro. I had only about 20 minutes to capture the Andromeda Galaxy before it hid behind the mountain. The data (40x30 s) was collected between 22:40 and 23:02 UT. Once captured, I turned off the tracking and proceeded with capturing the foreground (10x10 s). The foreground mountain was conveniently illuminated by a waxing gibbous moon that night. Both the background sky and the foreground images were calibrated, stacked, and processed in PixInsight, followed by blending and final polishing in Adobe Photoshop. The final image was resized to 40% and cropped.
So, I gather that Andromeda really would appear this large in the sky if our eyes were sensitive enough to detect its full extent.
In other news, this link to "virial mass" from the Wikipedia page
about M31 quickly lost me in a thicket of formulas - yikes! - https://en.wikipedia.org/wiki/Virial_mass
In astrophysics, the virial mass is the mass of a gravitationally bound astrophysical system, assuming the virial theorem applies. In the context of galaxy formation and dark matter halos, the virial mass is defined as the mass enclosed within the virial radius rvir of a gravitationally bound system, a radius within which the system obeys the virial theorem. The virial radius is determined using a "top-hat" model. A spherical "top hat" density perturbation destined to become a galaxy begins to expand, but the expansion is halted and reversed due to the mass collapsing under gravity until the sphere reaches equilibrium – it is said to be virialized. Within this radius, the sphere obeys the virial theorem which says that the average kinetic energy is equal to minus one half times the average potential energy, ⟨T⟩ = ½ ⟨U⟩, and this radius defines the virial radius.
And that's as far as I got.