I have a 15.3hh-16hh Welsh x TB, and she gets one haynet filled with about 3/4 slices of hay in, but she swells up like a ballon if you give her too much (she's a fat little pony at heart!) so if you had a TB or something you may want to give them more.
Unless the horse is overweight, then I would give as much hay as they can eat overnight. If you need to know this by weight, then weigh the amount every night and keep increasing it until there is a little left in the morning. Then subtract the weight of that left in the morning from how much you gave the night before and you will then know the optimum amount to put in each night!