This is easy math, fortunately. 28mm is 1/6th larger than 24mm, and correspondingly, your selfie taken from the same position will be 1/6th wider. If you stay in the center of the frame, you gain an additional 1/12th on each side, or, expressed another way, about 8% more on each side. 

For more, see http://photo.stackexchange.com/questions/9133/how-can-i-visualize-or-simulate-the-effect-of-different-focal-lengths