They vary in size depending on the region and who is making them for what purpose. They are made in India (where they may be called roti and also in Turkmenistan, in East African countries including Uganda, Kenya, and Tanzania;, and in West Africa nations such as Ghana.
Most homemade chapati are about the size of a taco tortilla because most home kitchens don’t have a tava (flat frying pan used to cook them) larger than that. Commercial kitchens can often produce a larger range of sizes.