You won't find a simple 'serving size' on the bag, because horses come in too many different shapes, sizes & requirements for that, but there should be a chart to give basic ration info. Just remember the amount on the pack/website for that is almost invariably too high... them wanting you to buy more of their feed & all. There is also a book available with precisesly that sort of info, but sorry, can't recall what it is.
As for calories, that info should be available on line too, or you could email & ask, but interestingly I recently read an article about calories of feed and there is a list of feeds that are commonly under- or over-estimated. I don't have it here to check, but I do recall that calorie content of beet pulp was around 20% higher than estimated!!