What postures are good to decrease the size of my belly?

What postures are good to decrease the size of my belly?

You want to practice yoga postures that build core strength, holding them as long as comfortable, and if possible engaging Mula Bandha in the postures.
The postures that build core strength are:
upward boat
upward facing dog
warrior 1, 2, and 3
triangle
warrior angle
locust
cobra
camel
bridge
bow
boat

To engage Mula Bandha: squeeze the muscles between the bottom of the pubic bone to the bottom of the tail bone, and focus on pulling up those muscles in towards your abdomen, especially focusing on contracting the perineum. Hold mula bandha as long as comfortable while practicing the above postures. When you get tired holding mula bandha relax and reengage when you are ready.
Building core body strength takes time, so start slow and be patient, and enjoy the process.

Ahimsa, the yogic practice of non violence must be adhered to when engaging in the practice of hatha yoga. Respect your body's limitations and inner wisdom, if something feels wrong or dangerous, please do not do it.

Please consult your health care practitioner before starting a yoga, pranayama or other exercise program.