It is very safe to feed whole. Horses can easily chew it in its whole foam and the stomach acids diegest the feed before it can turn toxic. Boiling it is toxic for humans, and grinding it will release toxins if you add water or oil when feeding it. So, the safest way to feed it is WHOLE!!
Flax has two elements that when combined, create cyanide. It only poses a very mild toxicity to horses if you grind it at home, then add water or oil to mix it with their feed. If it's milled (pre-ground) it is safe to add liquid to. If it is whole, it is safe to add liquid to. If you boil it, you release the toxins in your kitchen (for you to breathe) and you boil out much of the beneficial fatty acids and nutrients found in raw flax.
You can feed up to 1 lb of flax a day, by weight, with no problems. Though like everyone else mentioned, 1 cup a day (about 4-6 oz by weight) is the most common amount to feed.