I am not an expert but, if the pipe is that big, maybe it needs to be so water will not go over the road in a large rain event. With that in mind, you should keep what ever you are going to block the water with to have an opening with the same square footage as the pipe. So if you have a 5 foot pipe you should probably have a 5 foot box type riser. It should be sized by what your 100 year rain event is for your water shed is.
