I second upnover. Good explanation!
For trot poles, I usually measure out a little over 4 feet, using my feet heel to toe (I don't carry around a measuring tape!). That's usually a good place to start, and then you can adjust them shorter or longer as needed for your horse's stride.
If you want to use a trot pole in front of a jump, I believe it should be 9 feet from the base of the jump. Just in case you needed to know...