I thank you for answering all my questions and making everything clear !
Stellarium states that NGC 6905 has TIM 12, SB 12.99, and size 1'42", curious.
Well, this is due to the many discrepancies in size that can be applied to some objects.
Usually, the size is determined to the magnitude 25 (per square arc-second) isophote (line of equal brightness), and this is often a little deeper than we can see. As a result, many, if not most, deep sky objects are smaller in our scopes than the size listed in atlases. We actually see most of the brightest portions, but less of the dimmest portions.
But many deep-sky objects continue to get bigger as the isophote gets fainter beyond magnitude 25.
You probably have seen M31 quoted as being 3 degrees, or 3.5 degrees. Really deep photos show the galaxy is 5 to 5.5 degrees, and possibly much larger.
It has even been discussed that the halos of the Milky Way and M31 may already be in contact (!), and the halo of M31 extending out to 15 degrees. We don't see that--it is inferrred from studies of similar galaxies. Recent work suggests the Milky Way has more mass in its halo and M31 has less, while M31 has more stars.
So, just exactly how big is M31?
The same is true of planetary nebulae. Before the "poof" that sent out the large chunk of material we see illuminated, these dying stars may have sent out many other "poofs" of gas from their atmospheres.
M57 is usually described as 1.5' x 1' in size, and, indeed, that is what most amateurs see.
But deep photos of M57 ( http://astrophoto.com/Ring.htm ) show it to be more like 4' x 4', and really deep photos show it larger still.
So it is with NGC6905. It's normal size is 0.73' x 0.63', so its SB is brighter than the 12.0 TIM.
But, long exposures show it has large extended wings not normally seen or photographed ( http://de.wikipedia...._-_RHaBOIII.png ), doubling the size in that direction, and when that extra size is taken into account, the SB lowers significantly.
All I can tell you is that this one is fairly bright in a 4" refractor in dark skies, so it should be visible in brighter skies up to a point.
The part you'll see will be <1' across, so have a fairly high surface brightness (at least of the part you see).
In essence, the size of any deep sky object will be the size that you see. And that will vary with aperture, darkness of sky, etc.
Just, alas, another reason why magnitudes are only approximate things.