I came across this image which purports to show what the size of the Andromeda galaxy would be in the night sky if we could see its dim stars with the naked eye. Is this image reasonably correct with respect to size?
Unfortunately, galaxies don't have a neat edge (and the Moon is not at a fixed distance) so your question is difficult to answer in absolute terms.
However, a commonly quoted diameter for the disk of the Andromeda galaxy is something like 70 kpc (about 220,000 light years), which at a distance of 750 kpc (about 2.4 million light years, e.g. see Andromeda galaxy) gives an angular diameter of about 5 degrees, which is about 10 times the diameter of the full Moon.
Thus I think the picture is reasonably justified.
On the other hand, is it realistic to expect to be able to see Andromeda this big? I'm not so sure and have asked a linked question about this. For example, this (well-known) picture from the digitised sky survey shows an Andromeda disk that is less than 3 degrees across!