Your dog may be arching his back due to discomfort or pain, such as a spinal issue or muscle strain. It is important to consult with a veterinarian to determine the underlying cause and provide appropriate treatment.
Like anything, you will improve by getting more practice.
but some things that could help are; turning your feet out,
trying to keep you legs as straight and tense as possible, arching
(pointing) your toes, pulling back our shouders and standing as
straight as you can.
What size should your dog be?
That depends. If you live in a small apartment and keep the dog
inside, he should be small. If you live on a farm and want a dog to
chase the snakes, raccoons, and burglars away, he should be large.
If you live in a house and want to keep a dog in the back yard, he
could be somewhere in the middle.