It is hard to estimate without knowing all the data necessary to do the calculation... in order to calculate you need to know the 100 years storm precipitation for your area, the stream basin area, the upstream volume and the flow of that volume... without these it is impossible to calculate... so if you don't know and won't find out then overkill it, perhaps have three pipe and have the road build as demonstrated below so if it's overflow it will flow on each side of the culvert on the road.
other than that, my opinion is most of the concrete work seems over kill.. thrench out 12 inch below the culvert grade and back fill with A gravel, compact, back fill the rest with A gravel and compact every foot... if you want a up stream and down stream concrete bulkhead fine but the rest is over kill.
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