You can measure the size and slope of your gutters by using a tape measure, a chalk line, and a level. You need to measure the length of your roof edge where you want to install or replace your gutters and mark the starting and ending points. You also need to mark the slope or pitch of your gutters, which is the angle at which they will drain the water. The recommended slope is 1/4 inch per 10 feet of gutter, which means that for every 10 feet of gutter, the endpoint should be 1/4 inch lower than the starting point. You can use a chalk line or a level to mark the slope on your fascia board.