I doubt that there is much you can do. PowerPoint shares the Organisation Chart sub-program with other members of Office. Newer versions have an 'improved' version, but all versions attempt to fit all the boxes that you define into the slide /screen area.
As you create more boxes, the space available for each one decreases, as does the size of the text within each box.
The only sensible solution is to create less boxes per slide, or type fewer characters per box.
Consider showing a part of the organisation over several slides - for example, with a typical pyramid hierarchy, the first slide might show the detail of the top two layers, with lower layers represented by smaller coloured boxes with no text. The next slide(s) will show the detail of each coloured section.
There are 3rd-party Organisation Chart programs available that will give you a lot more flexibility, and allow you to cut and paste into PowerPoint, but they tend to be quite expensive for occasional use, and don't really overcome the problem of too much information on one slide.