I enjoy fantasizing only human/human UB, but visual material is hard to find. I fantasize things that hint at reality, not 'actually' doing the real UB.
Visual material to use as an aid to your Partial Unbirth or for your Total UB Fantasy will have to wait on available material. Just seeing a pelvis size required for a usual size adult head inserted is easy to locate if you don't mind extrapolating human/human over to human/other-species like above, or even here: http://www.dailymotion.com/video/xfnx85_acnecure_fun
Human visual material that I know about is limited, and mostly limited to a size that only a small size adult head would fit inside of -- like this motion gif. You can easily see that this pelvis is only slightly larger than average. But if you fantasize this same pelvis enlarged to one of the the rare disproportionate and enlarged Justo Major types, it becomes easier. Doing that, this same insertion could easily be large enough for a average or big adult head size.
If you don't mind fantasizing to an extremely remote possibility for doing a small Total UB size -- one like in my TUB story -- then a very much larger pelvis would have to be fantasized -- and you could fantasize her belly outline to look something like this girl kneeling on a mat -- with a internal partner bulging her lower belly. With such a gigantic pelvis, she would have to practice increasing her vaginal size and depth into her lower belly by long term stretching and allow maybe 30 inches of vaginal depth. A fetal position for an adult using 35 to 40 percent of their standing height allows the insertion. Notice how deep vaginal fisting makes these belly bulges show in a very small girl in a moving gif file -- located the bottom of this post -- and then compare what a whole adult body type bulges would show in the following gigantic TUB picture. This TUB picture shows a thick layer of slick white cooking lard in and around her huge vaginal opening ring and the inside partner adjusting his breathing tube with his right hand: download/file.php?id=75475&mode=view
It is included in my story's zip file download/file.php?id=75473
To see the vaginal volume some women with a common type pelvis can have, see: http://p2.xhamster.com/000/003/853/926_1000.jpg
. Watch the rectangular fluorescent light reflection spot seen just below her clitoris as it moves to your right. This woman has practiced enlarging her vagina with an inflatable balloon over some extended period (possibly months). Notice between the third and last frame in this set of serial frames it only takes her about a one second interval to push out this adult head size ball. This short time interval shows that 'all' the volume of the head sized balloon was once inside her vagina. If her pelvis were larger, to allow more volume or weight, her internal potential volume size goes upward as the cube of linear pelvis enlargement like: (larger pelvis linear size/her average pelvis linear size)^3 multiplied times her max shown vaginal internal volume or weight -- so just the enlarged linear ratio cubed is then multiplied times original internal weight size.
The result of this math shows what a large pelvis insertion could hold in terms of greater volume or weight. How much does a head weigh? Then the calculation would be: (big pelvis width/average pelvis width)^3 X head weight = weight that the big pelvis might contain after practicing for a similar time span -- like the above lady practiced. Using this simple math, the largest pelvis on Earth might not contain much more than a 2.5 linearly increased pelvis size ratio. Then this 2.5 ratio cubed, and multiplied times a head weight (or 2.5 x 2.5 x 2.5 x head weight) -- would be her similar internal size (using the weight of the head sized object that this lady could push out in the last frame of the picture). e.g. for 10# head -- 2.5 x 2.5 x 2..5 x 10# = 156.25#. In a similar manner, a linear 2.7 times as large would allow a huge 20 times the volume to fit into her vagina.
The giant size pelvis woman's internal size would also depend on whether she spent more or less time practicing her internal enlargement (the woman in my story practices internal enlargement for years) -- but for the above size and practice period, the huge internal space would be well over 100 pounds potentially insert-able, inside the World's largest possible opening.
From a practical standpoint you would have to view a half billion women to just see one that had this 2.5 + times average size pelvis -- and likely she would ignore you -- in the unlikely event you found her.
Below, the head size and depth would only be suitable for a smallish adult head.
If you have a small head, I certainly would NOT recommend you volunteer to sit beneath her -- in this same spot -- as she lowers and raises her weight.
Even if she slowed down a lot and raised up a little higher -- to let you get your breath between strokes -- in time the friction might make your ears smart. lol