Typically, if you are “overweight” you can multiply your age’s RDI by 1.5, and if you are “obese”, you can multiply your age’s RDI 2-3 times, and these numbers are an idea of what your daily recommended amount should be.[3] For example, the RDI for a 30-year-old is 600 IU. If that 30-year-old is overweight, then they should be taking close to 900 IU daily. If that 30-year-old is obese, they require 1200-1,800 IU daily. For an adult, doses up to 4,000 IU are considered safe.[4]
http://blog.vitaminddrops.com/myth-reality-vitamin-d-dosing-body-weight/ suggests for Vitamin D3: