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A good rule of thumb is to leave between 25 - 30 centimeters from the chair seat to the underside of the tabletop. This will give you enough space to sit with or without your legs crossed–and feel comfortable.
Standard armrests should be at about the same height as the point of your bent elbows. Since there are great variances in individual arm lengths and body proportions, we can't give you the exact measurements, but the arm rest should be around 15 centimeters above the chair for an average person.
Your seat should be wider than your hips, to allow space for movement and clothing. A seat 5 centimeters wider than you are is ideal.