What size compression garment (girdle) should I buy after my Tummy Tuck?

You should discuss this with your plastic surgeon who will be performing your tummy tuck.  A very tight compression garment can compress the blood supply to the tummy skin after tummy tuck and can result in skin necrosis or loss near your incision.Personally I use a gently tightened abdominal binder for comfort and to hold dressings in place and avoid a very constrictive tight compression garment until healing and evidence of good blood supply occurs usually at 2-3 weeks.

Your surgeon should be able to advice you and his staff will measure you and order it for you. There are many vendors out there, but i will let my surgeon's staff handle this matter. A good garment is a must after this surgery. It impacts your recovery  significantly.