I can only speak from my own experience --

Avoid 1/2". It could work for supplying the restroom or a towel washer, but for anything else, it's too restrictive. The best reason to avoid it altogether, is so you don't have to stock 1/2" fittings and pipe for repair jobs in the future. 3/4" is fine for supplying the wash vat, the parlor washdown hoses(probably need 1" if you're supplying a booster pump), and any return lane water troughs. As other posters have said, use 1" for any trunk lines from the pressure tanks. with 3/4" for most of the branch lines.

Don't direct bury water lines in the concrete floors. For under-concrete lines, cement in gently curved lines of black poly pipe, and thread smaller poly or pex through those. For lines within the utility room, lines to and within the milkroom, and lines to and within the parlor, just surface mount everything. Don't try to hide or bury waterlines in these places, since these lines need to be accessible for fast repairs in the future.

Our dairy uses 1" and 1 1/4" black poly for all underground purposes, and 1" and 3/4" CPVC for all surface-mount indoor pipes.  

Most importantly, size your pressure tanks properly.  You want enough useful capacity to supply the wash-vat while you're using wash-down hoses, keeping in mind that the cows will be drinking extra at the end of milking as well. Remember that the nominal capacity of a pressure tank is not the useful capacity.  An 80-gallon tank might give you 20 gallons of high pressure storage. (that percentage is a guess. Google it.)