You have to remember that the size of an object is in function of (a) aperture and (b) whether or not the catalog/database/map measurements were made photographically. A large scope using a camera will record a much bigger galaxy (any galaxy, really) than a smaller scope operated by a visual observer. The large photographic instrument will detect not only fainter stars, but a lower density of stars, which means a larger measurement will go on to the map. In terms of its actual boundaries you *can't* see the entire Andromeda galaxy and m32 and 110 in the same field of view.
Incidentally, you also get perceived variability depending on sky transparency. Go high into the Sierra and you'll get a bigger Andromeda galaxy.
The aperture is not a trivial issue. I *cannot* fit the Perseus double cluster into a single field of view on my C14. But when I center the C14 on the "empty space" (NOT!) between the two main clusters I actually see more group members than I see in both the main clusters in my 4" refractor at 17x in a four degree field.
While we all understand what you're talking about, your discussion of these issues will gain ENORMOUS clarity if you learn to think in terms of degrees of arc, arc minutes and true field of view, which is a function of the eyepiece focal length and the scope's focal length, etc. It might take a day or two to get the hang of it but then instead of writing:
I know someone in my club with a 10" dob can't see the whole Pleiades, but he also does not own a 32mm Plossl.
You might write:
I know someone with a 10" f/4.5 dob who can't see the whole pleiades, but his widest eyepiece is [for example] a 20mm plossl"
Then the busy minds here might get down to it:
Ten inches is 254 mm
Times 4.5 is 1143 (or 1200 fl family as you wrote)
The plossl is probably around 50 degrees "afov"
So we're talking about 1200/20=60 x magnification. For true field 50 degrees / 60x = 0.83 degrees
converting that to arc minutes: 60 arc minutes * .83 = 48 arc minutes
Now taking that known parameter of the eyepiece-scope combination, we look at a Deep Sky Atlas 2000 or software of any variety, and we see that this is inherently smaller than the object, never mind whether photographic, binocular, or whatever. In fact some software lets you plug in your eyepiece and scope combo and plop a circle of the correct radius on your target so you can see what you will see in the scope.
Now if your friend breaks down and spends thirty bucks on an inexpensive 32mm plossl, he is going to have 1200/32=37.5x, and a plossl will give approximately 50/37.5x or 1.33 degrees, or 60*1.33=120 arc minutes which is very close to what havasman just posted above.
And that is in fact the limit of the hypothetical f/4.5 with a 1.25" focuser. Change the focuser (and probably the secondary too), or give a scope with a longer or shorter focal ratio, and all these factors will change; and also as a function of aperture, with which *focal length* changes even if *focal ratio* does not. A thirty inch f/4.5 Newtonian telescope has a field of view almost as tight as an f/11 C14 (notorious for its narrow field of 40 arc minutes, tho' I love mine).
Now I don't mean to drown you in these arithmetic ratios which you seem to be more or less aware of because of your stipulation of 48 to 50 inches of focal length in your various ratios and mirror diameters.
But it remains the case that a 6" f/8 is gong to bring in A LOT LESS ANDROMEDA galaxy than your 12" f/4, so there is gong to be less there to see, but maybe framed better. The owner of the 12" f/4, in measuring the length of the galaxy, will come up with a *longer galaxy* than the f/8 6".
If you put the six inch scope in the Sierra and the 12" scope in the Adirondacks with 95% relative humidity, all bets are off. The 6 inch may bring in *more* Andromeda compared to the 12".
The question of framing is also somewhat distinct from a good view. First of all, there are many more objects available to the larger scope. The Perseus Double cluster can't be squashed into a c14 but the C14 provides a wonderful tight framing of NGC 7789.
NGC 7788 and NGC 7790 are as gratifying in an f/11 c14 (at 100x with 40 arc minute tfov) as the Perseus Double cluster is in a four or five inch f/6 refractor with a 3.5 to 4.5 inch fov.
There are so many gorgeous open clusters out there that the question of how to frame the Pleiades loses interest, really, once you know your way around the OCs suitable to your aperture.
There is the additional issue of learning-what-the-scope can do. I can frame the Veil Nebula in my 4" f/6.5 refractor and see the whole thing. It is quite the sight to see the circumference of that supernova remnant. But in my personal experience the intricacy of the part of the Veil known as NGC 6992 (aka the "Network Nebula") is much more engrossing, with the added bonus that when you get tired of taking it in you can use your paddle to scoot along the full length of that portion of the Veil, and then move on to the other one.
For these reasons I see in your query a wide variety of technical and aesthetic issues that are, as it were, unspoken, and I hope you don't mind my going on at length.