George Stephenson, the railway pioneer, has been said to have carved the shape out of a turnip and asked the bricklayers to "make it like that" (with a strong Gordie accent, of course).
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
That is a very open-ended question. You will need a lot of information to start with, the main ones being: expected peak flow rate, downstream water level and permissible upstream water level. You need to make some decisions regarding culvert shape, (circular pipe, rectangular, single cell, multi cell, etc.)
If you could elaborate on some fo your requirements?