If you have bag storage make sure it's easy to load bags onto the carts in the barn itself. Also, like driving ranges, I've never seen a cart storage area that was "too big"!
If you provide towels on the carts you'll need a washer/dryer nearby, if you have sand buckets you may want an area to store the sand--large trash can works well, as well as the bottles and back ups.
Small area for pencils, scorecards, tees. Also a wash area is nice inside. On the days when its raining and the carts look like crap and you need to clean them it's nice to be able to hose them off and clean them up out of the storm!
Make sure the ceiling is high enough so you don't bump your head on the chargers and have hooks in the ceiling to hold the cords when you unplug them. (Plenty of accidents where the cords catch in the back of the cart and get pulled down!!) Use zip ties or something like them to hold batteries in place on their pedestals.
It's nice to be able to back the carts in each night so you can pull them straight out in the AM. Either you have enough space to turn around inside, wide enough lanes to back in or two entrances/exits so you can pull in and have the cart facing out ready to go.
But, don't put a desk or chairs in there--the cart guys will stay inside and not be where they should waiting on the golfers--don't make it too comfortable back there
Lastly, the cart barn door is open all day. I use those walk in coolers plastic pieces in the winter to help keep the heat in. For walk-in coolers they are ceiling to floor plastic strips about 8 inches wide that overlap one another--they help trap the cold air inside. In the winter these work great at keeping the barn warm (and the carts and batteries as well).
Good luck and remember--bigger is better in this case.