Basically, if you want the answer without climbing up a ladder, you'll need some math and a few tools.
Let's take the basic example of the height of your home from the ground to the eaves (which is good to know because you'll need an extension cord at least that long to get to the ground, plus however much more you'll need to get to the outlet). First, measure out the distance along the ground to some point of observation. A click wheel is good, or you can pace it out if you know your average stride length. Then, you'll need some tool to measure a vertical angle. You can improvise a sextant (which is the true tool for the job) using a protractor and weighted string. Measure the angle from your point of observation up to the eaves.
Now comes the trig. The tangent of an angle theta is defined in terms of the right triangle that includes that angle; the tangent is the ratio of the side of that triangle opposite the angle, to the side of the triangle adjacent to the angle.
In other words, from your point of observation away from the house, the height of your house can be determined using the distance you are away from it, and the angle at which you have to look up to see the eave of your roof. You have the angle and the length of the adjacent side: tan(theta)= O / A, so O = A * tan(theta). Now, because you were probably standing when you took the angle measurement, add your height to the answer.
You can use similar methods to find the length of a horizontal line on the roof, the length along a gable, etc etc.