I agree with Dave, too. Leave enough room around your waterers and make sure there is a clear drain path. One of our barns is sloped to the reception pit, that makes cleaning a bit easier. If possible, lay it out for scrapers even if you don't put them in right away.

Don't skimp on rebar, it's pretty easy to crack a curb with a skidsteer and you want it to stay together.

The advice to watch closely when they finish the floor is good, some contractors will cut corners a bit if you are not watching. Mostly because they don't understand how important it is.

Has good insulation under the roof been mentioned?