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As an architect, I can tell you that an abutment is essentially the same thing as a foundation, such as the footings under your house. The main function is to distribute the weight of the bridge and its traversers into the ground, as well as to hold back the loose soil either side of the span. The size is determined by a variety of factors, such as the soil bearing capacity, depth of bedrock, depth of the span, slopes, etc. In general, the concrete must be poured down below grade to at least below the frost line, which is how deep the ground freezes during winter. In modeling terms, it would be simplest to approximate this by providing extra abutment and then "hiding" it underground, as you bring the scenery up to it. This is much easier than trying to fashion the abutment to fit the profile of the scenery, and also ends up looking better. As for width, this mainly depends on what the abutment is holding back. If the approach to the bridge is over a fill, then it will be loose soil that mounds up at a certain angle-of-repose (think how sand in an hourglass piles up). The abutment and its wings must be wide enough to retain all of this fill, which makes it pretty broad at the base. At the top, of course, it must be wide enough for your roadbed. Ties that are wider than the rail guage and the ballast beneath them that spreads even wider are serving the same function of distributing weight, so this determines the soil width at the top of the fill; off the top of my head, I'd guess that 20 feet of soil fill is sufficient for a single track. If the angle of repose is essentially a 1:2 ratio (rise:run), and your abutment stands 10 feet tall, then at the base it will have widened to 60 feet (10 ft rise = 20 ft run each side). 1:2 is a very optimistic angle of repose, but selective compression may require us to go even steeper than that. As for wings that bend around, I think that is just a trick to minimize width without compromising the angle, as well as a way to prevent stream flow from cutting behind the abutment. I might be wrong, though.
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