You can buy a small body guitar or a 3/4th size classical guitar from a store. Go to a local store and see how different-sized guitars suit your body while sitting. Consider the guitar body's thickness which ranges from 4.5cm (1.8 inches) to 10 cm (4 inches). Something around 2-3 inches is ideal for most beginners. It affects the "size" of the sound but it would hardly matter as a beginner.