A growing 650+ lb. heifer is going to eat nearly as much as an adult cow because she needs feed to grow on. If she is getting some grain along then not so much. We figure 1 5x5 roll per cow per 20 days here in Va for the winter. That is averaging the ones with calves and the ones without. I would want to have at least 4 rolls on hand minimum, 1 per month. Keep the ones covered with a tarp to protect them and then when they are uncovered and put out to be fed there will be a little less waste. Unless you are in a drought area, then hay should be not too expensive. Don't know what kind of hay you are looking at.
Small square bales would definitely be less waste, but you have to have a place to store them and then if you get bad weather, can you get around to feed them if it is mud or snow.? It sounds like the hay will be fed out in pasture not in a barn area.