Document is 15.1 mb-needs to be 5mb What do I do to make it an acceptable size?

If this is a picture in Photos, you can export it and adjust the image and/or document size. Select the picture, then under the File menu choose Export/Export 1 photo. A dialog box will open with options for choosing the format, picture quality (in jpeg format, anyway) and picture size - that is, pixel dimensions. When you click the "Export" button, you can navigate to the folder where you want it. You might want to experiment with different combinations, depending on how the picture will be used.