How long an extension ladder do I need

Personally, I hate ladders with a passion. :) Heights are best left to birds, squirrels, etc. Having said that...

A ladder stabilizer can often help. This is a bracket that attaches to your ladder. It makes the ladder much more stable when leaned against the house. A nice thing about the ladder stabilizer is it lets you lean the ladder on the wall of your house, NOT against the edge of your gutter, possibly causing damage to the gutters.

Ladder-Max Stand-off Stabilizer

Next, make sure the ground is flat and solid under your ladder's feet. If the ground is not so, I made a wooden wedge to place under one of the feet of my ladder to make sure the ladder will be happily placed. The bottom of that wedge has spikes sticking out of it, so it will not slip on the grass. You can also buy extension ladders with one leg than can be adjusted in length to account for uneven ground.

To climb onto the roof, you want to run the top of the ladder past the roof, so it extends far enough that you can hold on when you step on and off. I'd suggest it should be at least 3 feet past the edge of your gutter, and 4 feet is better. I also have a helper hold onto the bottom of the ladder to stabilize it. A helper is a good idea in general anyway. One thing you don't wish is for the ladder to fall down when you are standing on the roof, leaving you with no way down, and nobody around to put the ladder back in position (or, as my wife might add, to call 911 when necessary.) A picture is worth a thousand words though:

Getting back onto the ladder off the roof is merely a matter of reversing your steps, as you hold onto the top end of the ladder with your hands.

Obey this setback rule: keep the base of the ladder at least 1 foot out from the house for every 3 feet in height. (Yes, many say to use a 1/4 ratio, but I often tend to go for 1/3.) So if the top of the ladder touches the house at 15 feet up, the base wants to be between 4 and 5 feet out. The trick is to not have it at too steep of a pitch, as then it would too easily tip away from the house - a bad thing especially if you are on it. The idea is that when you are standing on the ladder, you always want the center of gravity (of you plus the ladder) to be between the foot of the ladder and the house.

If I have an extension ladder standing on my deck, I'll often screw down a 2x4 cleat outside of the feet. This prevents the feet from moving/slipping out when I am on the ladder. On a grass or dirt footing, the feet of an extension ladder can swivel into what are effectively spikes that can be set firmly into the ground. These prevent the base of the ladder from moving out from under you.

Other rules - don't climb a ladder if you can avoid it when it is windy, or if you are even remotely tired or not completely focused on the matter at hand. If you are very uncomfortable with climbing ladders, money solves all problems here, as you can always hire even small jobs out.

Here are a few links you might find useful:

How to Climb a Ladder Safely

The Right Way to Set Up and Climb a Ladder

The distance the ladder base should be from the wall is 1/4 of the vertical height of the ladder contact point. The side load will be minimal. However with the ladder against the rafters it will be difficult to position the insulation and not have the ladder in the way.