how high do i have to raise my bicycle seat?

hiyo ^^ im just wondering how high i have to raise my bike seat in order to get (or try to get) some height gain, i've realized that if i raise my seat too high, its relatively dangerous and i have trouble braking and touching my toes on the ground. so do i have to lower the seat or increase it in height. im not really sure as to when we're supposed to be stretching our legs. are we supposed to be struggling to make a full revolution on the bike pedals?

Thanx for helping in advance,

~haikin