For the traditional Omani pants, called sirwaal, 2 meters (or 3 if you are VERY tall) is sufficient.
For the shawl, two meters.For the Dhofari tunic with its train, pictured above, 3-4 and 1/2 meters is required, as well as any subsequent linings.
The dress style pictured above is called a "wael". For a "wael" including a "warqiya" shawl, you will need around six meters of your desired fabric. Printed cotton is the most common, but also beautiful sheer silk chiffons for more formal occasions. Often sheer if made of one layer, sirwaal pants may be worn with them, or six meters of cotton or silk lining may be purchased. I own many cotton ones and wear them without abaya in many places outside of Muscat.
It usually costs 3 rial for a tailor to sew you a wael, 4-5 rial for a Omani outfit with warqiyah, tunic, and sirwaal, and 5-6 rial for a Dhofari dress in Mutrah, but you can barter down from those prices. Any embroidery or beadings and the price goes up and the time is increased for your order. Some are sent away to Pakistan and take two months to complete. Keep this in mind.
Prices in Al Bahrimi and Salalah are cheaper.