trotting/canter poles

Depends on the size of the stride. I do 4 pigeon steps for trotting poles, and one pole length for canter poles. I have a 16hh but he does have a small stride. Worth setting some out and playing around with it until it's comfortable, and then measure.
 
Impossible to answer as it depends on your individual horse's stride length and whether/how you are trying to change that.

Can't you find an experienced person/instructor to help you set up trot/canter poles and then to adjust them?

It does depend on what you are trying to achieve but if you want a starting point I use the average of a 12 ft canter stride, which is what jumping courses tend to work with, I bring in a little shorter to start with for a smaller horse, unless they are very choppy in their stride trot poles will fit in by being half the canter distance so you can either fill in the gaps to have a pole every stride or leave them at canter distance and trot through them with a pole at alternate strides, so about 5-6 ft apart if using one every stride for trot work, a bit of playing about once you set them up is usually required to get the best use out of any polework.
 
I have 14.2( give or take) ponies and I use 1 stride between trotting poles and 3 between canter poles, I vary this at times too, to lengthen and shorten strides. If you are unsure, set a couple of poles out and lunge over them first so you get an idea of your horses stride.
 
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